Don't report a syntax error when an empty string is passed to ast_get_group.

Just return 0.

(closes issue #11540)
Reported by: tzafrir
Patches: 
      group_empty.diff uploaded by tzafrir (license 46)
	   -- slightly changed by me


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@95024 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -4596,6 +4596,9 @@ ast_group_t ast_get_group(const char *s)
 	int start=0, finish=0, x;
 	ast_group_t group = 0;
 
+	if (ast_strlen_zero(s))
+		return 0;
+
 	c = ast_strdupa(s);
 	
 	while ((piece = strsep(&c, ","))) {