Fix warn_unused_result attribute failure

On some libcs posix_memalign is defined with warn_unused_result. Check
its return value and handle it appropriately in this test.

Change-Id: Iaa7151fe101b0dfc04192229a9a8751e1f92507f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507037
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638388}

components/gwp_asan/client/sampling_allocator_shims_unittest.cc

@@ -135,9 +135,10 @@ MULTIPROCESS_TEST_MAIN_WITH_SETUP(
 
 #if defined(OS_POSIX)
   EXPECT_TRUE(allocationCheck(
-      [&] {
+      [&]() -> void* {
         void* ptr;
-        posix_memalign(&ptr, page_size, page_size);
+        if (posix_memalign(&ptr, page_size, page_size))
+          return nullptr;
         return ptr;
       },
       &free, &failures));