Fix a segfault in swarmvtab that could occur if there was an error in the SQL

statement passed to the constructor. Add other test cases.

FossilOrigin-Name: 6ce8b7ca62fcf97875395fc1a989179309e0abb48d4465658ef0d871434ea057

ext/misc/unionvtab.c

@@ -721,7 +721,7 @@ static int unionConnect(
     pStmt = 0;
 
     /* Capture the not-found callback UDF name */
-    if( argc>=5 ){
+    if( rc==SQLITE_OK && argc>=5 ){
       pTab->zNotFoundCallback = unionStrdup(&rc, argv[4]);
       unionDequote(pTab->zNotFoundCallback);
     }