zebra: fix use after free on RIB processing

After calling `rib_unlink` the variable `re` will point to `free()`d
memory, so don't attempt to use it after this point.

Found by Coverity Scan (Coverity ID 1519784)

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>

zebra/zebra_rib.c

@@ -1306,6 +1306,7 @@ static void rib_process(struct route_node *rn)
 									vrf),
 								vrf_id, rn);
 						rib_unlink(rn, re);
+						continue;
 					} else
 						SET_FLAG(re->status,
 							 ROUTE_ENTRY_REMOVED);