fix data race in at_quick_exit

aside from theoretical arbitrary results due to UB, this could
practically cause unbounded overflow of static array if hit, but
hitting it depends on having more than 32 calls to at_quick_exit and
having them sufficiently often.

src/exit/at_quick_exit.c

@@ -21,9 +21,10 @@ void __funcs_on_quick_exit()
 
 int at_quick_exit(void (*func)(void))
 {
-	if (count == 32) return -1;
+	int r = 0;
 	LOCK(lock);
-	funcs[count++] = func;
+	if (count == 32) r = -1;
+	else funcs[count++] = func;
 	UNLOCK(lock);
-	return 0;
+	return r;
 }