patch 8.2.2173: Vim9: get internal error when assigning to undefined variable

Problem:    Vim9: get internal error when assigning to undefined variable.
Solution:   Add error message. (closes #7475)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 606ce0c..db15cea 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -606,8 +606,7 @@
 	return FAIL;
     if (ufunc->uf_def_status == UF_COMPILED)
     {
-	int error = check_user_func_argcount(ufunc, argcount);
-
+	error = check_user_func_argcount(ufunc, argcount);
 	if (error != FCERR_UNKNOWN)
 	{
 	    if (error == FCERR_TOOMANY)