loader: Avoid use-after-free / use of uninitialized local variables

loader: Avoid use-after-free / use of uninitialized local variables

Per the valgrind output below, we were returning the pointer to freed
memory if none of the later conditional pointer assignments were
executed. This caused dEQP CI jobs to crash on certain runners,
presumably due to a double-free down the line.

Also, we were skipping to the out: label before the vendor_id & chip_id
variables used by it were initialized, resulting in broken
LIBGL_DEBUG=verbose output such as

libGL: pci id for fd 4: 51108f00:51108f00, driver radeonsi

Fixes: 5a545e355b23 "loader: always map the "amdgpu" kernel driver name to radeonsi (v2)"

==403== Invalid read of size 1
==403==    at 0x4AFD576: surfaceless_probe_device (platform_surfaceless.c:316)
==403==    by 0x4AFD915: dri2_initialize_surfaceless (platform_surfaceless.c:391)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:984)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:958)
==403==    by 0x4AF1EEC: _eglMatchAndInitialize (egldriver.c:75)
==403==    by 0x4AF1F3B: _eglMatchDriver (egldriver.c:96)
==403==    by 0x4AE9367: eglInitialize (eglapi.c:617)
==403==    by 0x1D99C9: tcu::surfaceless::EglRenderContext::EglRenderContext(glu::RenderConfig const&, tcu::CommandLine const&) [clone .constprop.57] (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x1DABB0: tcu::surfaceless::ContextFactory::createContext(glu::RenderConfig const&, tcu::CommandLine const&, glu::RenderContext const*) const (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x53EBD1: glu::createRenderContext(tcu::Platform&, tcu::CommandLine const&, glu::RenderConfig const&, glu::RenderContext const*) (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x53EFE9: glu::createDefaultRenderContext(tcu::Platform&, tcu::CommandLine const&, glu::ApiType) (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x1DE07A: deqp::gles2::Context::Context(tcu::TestContext&) (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x1DB5EF: deqp::gles2::TestPackage::init() (in /deqp/modules/gles2/deqp-gles2)
==403==  Address 0x56bd340 is 0 bytes inside a block of size 4 free'd
==403==    at 0x48369AB: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==403==    by 0x4B01767: loader_get_driver_for_fd (loader.c:464)
==403==    by 0x4AFD553: surfaceless_probe_device (platform_surfaceless.c:308)
==403==    by 0x4AFD915: dri2_initialize_surfaceless (platform_surfaceless.c:391)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:984)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:958)
==403==    by 0x4AF1EEC: _eglMatchAndInitialize (egldriver.c:75)
==403==    by 0x4AF1F3B: _eglMatchDriver (egldriver.c:96)
==403==    by 0x4AE9367: eglInitialize (eglapi.c:617)
==403==    by 0x1D99C9: tcu::surfaceless::EglRenderContext::EglRenderContext(glu::RenderConfig const&, tcu::CommandLine const&) [clone .constprop.57] (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x1DABB0: tcu::surfaceless::ContextFactory::createContext(glu::RenderConfig const&, tcu::CommandLine const&, glu::RenderContext const*) const (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x53EBD1: glu::createRenderContext(tcu::Platform&, tcu::CommandLine const&, glu::RenderConfig const&, glu::RenderContext const*) (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x53EFE9: glu::createDefaultRenderContext(tcu::Platform&, tcu::CommandLine const&, glu::ApiType) (in /deqp/modules/gles2/deqp-gles2)
==403==  Block was alloc'd at
==403==    at 0x483577F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==403==    by 0x4EE5E09: strndup (strndup.c:43)
==403==    by 0x4B010B1: loader_get_kernel_driver_name (loader.c:101)
==403==    by 0x4B016AF: loader_get_driver_for_fd (loader.c:462)
==403==    by 0x4AFD553: surfaceless_probe_device (platform_surfaceless.c:308)
==403==    by 0x4AFD915: dri2_initialize_surfaceless (platform_surfaceless.c:391)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:984)
==403==    by 0x4AF5EEA: dri2_initialize (egl_dri2.c:958)
==403==    by 0x4AF1EEC: _eglMatchAndInitialize (egldriver.c:75)
==403==    by 0x4AF1F3B: _eglMatchDriver (egldriver.c:96)
==403==    by 0x4AE9367: eglInitialize (eglapi.c:617)
==403==    by 0x1D99C9: tcu::surfaceless::EglRenderContext::EglRenderContext(glu::RenderConfig const&, tcu::CommandLine const&) [clone .constprop.57] (in /deqp/modules/gles2/deqp-gles2)
==403==    by 0x1DABB0: tcu::surfaceless::ContextFactory::createContext(glu::RenderConfig const&, tcu::CommandLine const&, glu::RenderContext const*) const (in /deqp/modules/gles2/deqp-gles2)

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>