intel/fs: Check for NULL key in fs_visitor constructor

intel/fs: Check for NULL key in fs_visitor constructor

Flagged by UBSan:

../src/intel/compiler/brw_fs_visitor.cpp:986:20: runtime error: member access within null pointer of type 'const struct brw_base_prog_key'
    #0 0x559fadb48556 in fs_visitor::init() ../src/intel/compiler/brw_fs_visitor.cpp:986
    #1 0x559fadb46db3 in fs_visitor::fs_visitor(brw_compiler const*, void*, void*, brw_base_prog_key const*, brw_stage_prog_data*, nir_shader const*, unsigned int, int, brw_vue_map const*) ../src/intel/compiler/brw_fs_visitor.cpp:962
    #2 0x559fad9c7cd8 in saturate_propagation_fs_visitor::saturate_propagation_fs_visitor(brw_compiler*, brw_wm_prog_data*, nir_shader*) (/home/daenzer/src/mesa-git/mesa/build-amd64-sanitize/src/intel/compiler/fs_saturate_propagation+0x61bcd8)
    #3 0x559fad9960a1 in saturate_propagation_test::SetUp() ../src/intel/compiler/test_fs_saturate_propagation.cpp:65
    #4 0x559fadd7a32d in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ../src/gtest/src/gtest.cc:2402
    #5 0x559fadd65c3b in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ../src/gtest/src/gtest.cc:2438
    #6 0x559fadd0af75 in testing::Test::Run() ../src/gtest/src/gtest.cc:2470
    #7 0x559fadd0d8a4 in testing::TestInfo::Run() ../src/gtest/src/gtest.cc:2656
    #8 0x559fadd10032 in testing::TestCase::Run() ../src/gtest/src/gtest.cc:2774
    #9 0x559fadd2ba0c in testing::internal::UnitTestImpl::RunAllTests() ../src/gtest/src/gtest.cc:4649
    #10 0x559fadd7df46 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) ../src/gtest/src/gtest.cc:2402
    #11 0x559fadd69613 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) ../src/gtest/src/gtest.cc:2438
    #12 0x559fadd2302e in testing::UnitTest::Run() ../src/gtest/src/gtest.cc:4257
    #13 0x559fadda2d61 in RUN_ALL_TESTS() ../src/gtest/include/gtest/gtest.h:2233
    #14 0x559fadda2c21 in main ../src/gtest/src/gtest_main.cc:37
    #15 0x7fe8f6748bba in __libc_start_main ../csu/libc-start.c:308
    #16 0x559fad9950f9 in _start (/home/daenzer/src/mesa-git/mesa/build-amd64-sanitize/src/intel/compiler/fs_saturate_propagation+0x5e90f9)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>