CheckStructHasMember: fix null deref warning (#15413)

Clang Static Analyzer is so smart that it reports a defect when this
intended null-deref is encountered.  Use sizeof instead which has no
runtime effects.

Modules/CheckStructHasMember.cmake

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