CheckStructHasMember: Avoid clang -Wall breakage (#15203)

With CMAKE_C_COMPILER=clang and CMAKE_C_FLAGS='-Wall -Werror', this test
breaks with -Werror,-Wuninitialized.  Fix this by getting rid of the
temporary variable.

Modules/CheckStructHasMember.cmake

@@ -69,8 +69,7 @@ macro (CHECK_STRUCT_HAS_MEMBER _STRUCT _MEMBER _HEADER _RESULT)
 ${_INCLUDE_FILES}
 int main()
 {
-   ${_STRUCT}* tmp;
-   (void) tmp->${_MEMBER};
+   (void)((${_STRUCT} *)0)->${_MEMBER};
    return 0;
 }
 ")