patch 8.2.3235: cannot use lambda in {} block in user command

Problem:    Cannot use lambda in {} block in user command. (Martin Tournoij)
Solution:   Do not go over the end of the lambda.
diff --git a/src/userfunc.c b/src/userfunc.c
index 7c96347..e008718 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1398,7 +1398,7 @@
 
 	// If there are line breaks, we need to split up the string.
 	line_end = vim_strchr(start, '\n');
-	if (line_end == NULL)
+	if (line_end == NULL || line_end > end)
 	    line_end = end;
 
 	// Add "return " before the expression (or the first line).