rst: Error code got hidden by successful core read

The ret is overwritten by core read sub-routine. Need
to reset it to -1 to keep failing in case of e.g. last
pid sysctl write.
Reported-by: Neal Becker <ndbecker2@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -947,6 +947,8 @@ static inline int fork_with_pid(struct pstree_item *item)
 	} else
 		ca.core = NULL;
 
+	ret = -1;
+
 	ca.item = item;
 	ca.clone_flags = item->rst->clone_flags;