patch 8.2.3141: no error when using :complete for :command without -nargs

Problem:    No error when using :complete for :command without -nargs.
Solution:   Give an error. (Martin Tournoij, closes #8544, closes #8541)
diff --git a/src/usercmd.c b/src/usercmd.c
index a4bbfd7..94ebb43 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -1019,21 +1019,15 @@
     // we are listing commands
     p = skipwhite(end);
     if (!has_attr && ends_excmd2(eap->arg, p))
-    {
 	uc_list(name, end - name);
-    }
     else if (!ASCII_ISUPPER(*name))
-    {
 	emsg(_("E183: User defined commands must start with an uppercase letter"));
-	return;
-    }
     else if ((name_len == 1 && *name == 'X')
 	  || (name_len <= 4
 		  && STRNCMP(name, "Next", name_len > 4 ? 4 : name_len) == 0))
-    {
 	emsg(_("E841: Reserved name, cannot be used for user defined command"));
-	return;
-    }
+    else if (compl > 0 && (argt & EX_EXTRA) == 0)
+	emsg(_(e_complete_used_without_nargs));
     else
 	uc_add_command(name, end - name, p, argt, def, flags, compl, compl_arg,
 						  addr_type_arg, eap->forceit);