virgl,vtest: Initialize return value

virgl,vtest: Initialize return value

Avoids:
Conditional jump or move depends on uninitialised value(s)
  at 0x9E2B39F: virgl_vtest_winsys_resource_cache_create (virgl_vtest_winsys.c:379)
  by 0x9E2725F: virgl_buffer_create (virgl_buffer.c:169)
  by 0x9E246D5: virgl_resource_create (virgl_resource.c:60)
  by 0xA0C1B9F: bufferobj_data (st_cb_bufferobjects.c:344)
  by 0xA0C1B9F: st_bufferobj_data (st_cb_bufferobjects.c:390)
  by 0x9F4ACE3: vbo_use_buffer_objects (vbo_exec_api.c:1136)
  by 0xA0C68C3: st_create_context_priv (st_context.c:416)
  by 0xA0C707A: st_create_context (st_context.c:598)
  by 0x9F81C6B: st_api_create_context (st_manager.c:918)
  by 0x9BBE591: dri_create_context (dri_context.c:161)
  by 0x9BB6931: driCreateContextAttribs (dri_util.c:473)
  by 0x4E97A44: drisw_create_context_attribs (drisw_glx.c:630)
  by 0x4E7C591: glXCreateContextAttribsARB (create_context.c:78)
Uninitialised value was created by a stack allocation
  at 0x9E2B249: virgl_vtest_winsys_resource_cache_create (virgl_vtest_winsys.c:342)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>