Use standard offsetof facility. Dereferencing a null pointer results

in undefined behaviour.

code/qcommon/q_shared.h

@@ -160,6 +160,7 @@ typedef int intptr_t;
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <time.h>
 #include <ctype.h>