rpc: fix logic bug

Spotted by Coverity.  If we don't update tmp each time through
the loop, then if the filter being removed was not the head of
the list, we accidentally lose all filters prior to the one we
wanted to remove.

* src/rpc/virnetserverclient.c (virNetServerClientRemoveFilter):
    Don't lose unrelated filters.

src/rpc/virnetserverclient.c

@@ -240,6 +240,7 @@ void virNetServerClientRemoveFilter(virNetServerClientPtr client,
             VIR_FREE(tmp);
             break;
         }
+        prev = tmp;
         tmp = tmp->next;
     }