target/target: fix clang static analyzer warnings

Change-Id: I23e6586be60915f21a7179a994a1ec93fb9b2c36
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5379
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>

src/target/target.c

@@ -1682,7 +1682,7 @@ static int target_call_timer_callbacks_check_time(int checktime)
 	 * next item; initially, that's a standalone "root of the
 	 * list" variable. */
 	struct target_timer_callback **callback = &target_timer_callbacks;
-	while (*callback) {
+	while (callback && *callback) {
 		if ((*callback)->removed) {
 			struct target_timer_callback *p = *callback;
 			*callback = (*callback)->next;