8338154: Fix -Wzero-as-null-pointer-constant warnings in gtest framework

Reviewed-by: ihse, dholmes, jwaters

Author: Kim Barrett

make/hotspot/lib/CompileGtest.gmk

@@ -57,7 +57,8 @@ $(eval $(call SetupJdkLibrary, BUILD_GTEST_LIBGTEST, \
         $(GTEST_FRAMEWORK_SRC)/googletest/src \
         $(GTEST_FRAMEWORK_SRC)/googlemock/src, \
     INCLUDE_FILES := gtest-all.cc gmock-all.cc, \
-    DISABLED_WARNINGS_gcc := undef unused-result format-nonliteral maybe-uninitialized, \
+    DISABLED_WARNINGS_gcc := undef unused-result format-nonliteral \
+        maybe-uninitialized zero-as-null-pointer-constant, \
     DISABLED_WARNINGS_clang := undef unused-result format-nonliteral, \
     DEFAULT_CFLAGS := false, \
     CFLAGS := $(JVM_CFLAGS) \

test/hotspot/gtest/gtestMain.cpp

@@ -336,7 +336,7 @@ static void run_in_new_thread(const args_t* args) {
 extern "C" void* thread_wrapper(void* p) {
   const args_t* const p_args = (const args_t*) p;
   runUnitTestsInner(p_args->argc, p_args->argv);
-  return 0;
+  return nullptr;
 }
 
 static void run_in_new_thread(const args_t* args) {