[NFC][Asan] Fix warning in test (llvm#155447)

After llvm#150028.

Warning:
```
asan_test.cpp:398:27: error: allocation of insufficient size '0' for type 'int' with size '4'
```

Author: vitalybuka

compiler-rt/lib/asan/tests/asan_test.cpp

@@ -395,10 +395,8 @@ TEST(AddressSanitizer, ReallocTest) {
   }
   free(ptr);
   // Realloc pointer returned by malloc(0).
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Walloc-size"
-  int *ptr2 = Ident((int*)malloc(0));
-#pragma clang diagnostic pop
+  volatile void *ptr0 = malloc(0);
+  int *ptr2 = Ident((int *)ptr0);
   ptr2 = Ident((int*)realloc(ptr2, sizeof(*ptr2)));
   *ptr2 = 42;
   EXPECT_EQ(42, *ptr2);