c11/threads: correct assertion

We should assert when either the function or the flag pointer
is null or we'll end up with a null reference a few lines later.

Currently unused by mesa thus it has gone unnoticed.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

include/c11/threads_win32.h

@@ -296,7 +296,7 @@ static void impl_tss_dtor_invoke()
 static inline void
 call_once(once_flag *flag, void (*func)(void))
 {
-    assert(!flag && !func);
+    assert(flag && func);
 #ifdef EMULATED_THREADS_USE_NATIVE_CALL_ONCE
     {
     struct impl_call_once_param param;