Allow FREE(NULL) to match standard C behavior

The previous implementation of FREE would call exit_failure() if passed a
NULL pointer. This caused fatal crashes in several core functions (such
as Link_download_full) that rely on FREE(NULL) being a no-op, especially
during first-iteration loop resets and error-path cleanups.

Aligning FREE with standard free() behavior improves overall system
stability and prevents minor logic patterns from triggering total
application failure.

Author: fangfufu

src/util.c

@@ -254,8 +254,6 @@ void FREE(void *ptr)
 {
     if (ptr) {
         free(ptr);
-    } else {
-        lprintf(fatal, "attempted to free NULL ptr!\n");
     }
 }