patch 8.2.2148: Vim9: crash when user command doesn't match

Problem:    Vim9: crash when user command doesn't match.
Solution:   Adjust command index. (closes #7479)

src/ex_docmd.c

@@ -3520,7 +3520,7 @@ find_ex_command(
 		++p;
 	    p = find_ucmd(eap, p, full, NULL, NULL);
 	}
-	if (p == eap->cmd)
+	if (p == NULL || p == eap->cmd)
 	    eap->cmdidx = CMD_SIZE;
     }
 

src/testdir/test_vim9_cmd.vim

@@ -701,5 +701,14 @@ def Test_cmd_argument_without_colon()
   delete('Xfile')
 enddef
 
+def Test_ambiguous_user_cmd()
+  var lines =<< trim END
+      com Cmd1 eval 0
+      com Cmd2 eval 0
+      Cmd
+  END
+  CheckScriptFailure(lines, 'E464:')
+enddef
+
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2148,
 /**/
     2147,
 /**/