If the asp lookup returns NULL, don't try to use it

If there is no corresponding asp, we set the result properly
but still fall through and attempt to use it.  Instead, just
continue.
diff --git a/agent/mibgroup/agent/nsTransactionTable.c b/agent/mibgroup/agent/nsTransactionTable.c
index 71f4188..31b9fe5 100644
--- a/agent/mibgroup/agent/nsTransactionTable.c
+++ b/agent/mibgroup/agent/nsTransactionTable.c
@@ -195,6 +195,7 @@
         if (asp == NULL) {
             netsnmp_set_request_error(reqinfo, requests,
                                       SNMP_NOSUCHINSTANCE);
+            continue;
         }
 
         /*