Don't use entry after free in the "already in lists" case

Return with 0 as it isn't an error.

MFC after: 1 week
CID: 1006085
Reported by: Coverity
Obtained from: Isilon OneFS (part of r493633)
Submitted by: Thor Steingrimsson <thor.steingrimsson@isilon.com>
Sponsored by: EMC / Isilon Storage Division

usr.sbin/bsnmpd/tools/libbsnmptools/bsnmpimport.c

@@ -693,6 +693,7 @@ snmp_import_table(struct snmp_toolinfo *snmptoolctx, struct snmp_oid2str *obj)
 		/* Same entry already present in lists. */
 		free(entry->string);
 		free(entry);
+		return (0);
 	}
 
 	(void) snmp_import_update_table(ENTRY_INDEX, entry);