log: guard free(NULL) in set_log_path()

free(log->path) is called before path has been set on the first
invocation, triggering a spurious WARNING from the custom FTLfree()
wrapper.  Add a NULL check to avoid the noise.

Signed-off-by: darkexplosiveqwx <101737077+darkexplosiveqwx@users.noreply.github.com>

Author: darkexplosiveqwx

src/log.c

@@ -144,7 +144,8 @@ static void set_log_path(struct log_fd *log, const char *path)
 {
 	if(log->path != NULL && path != NULL && strcmp(log->path, path) == 0)
 		return; // unchanged
-	free(log->path);
+	if(log->path != NULL)
+		free(log->path);
 	log->path = path != NULL ? strdup(path) : NULL;
 }