Fix chan_modem_i4l to be sure we use unsigned (bug #3412)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@4886 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_modem_i4l.c

@@ -310,7 +310,7 @@ static struct ast_frame *i4l_handle_escape(struct ast_modem_pvt *p, char esc)
 
 static struct ast_frame *i4l_read(struct ast_modem_pvt *p)
 {
-	char result[256];
+	unsigned char result[256];
 	short *b;
 	struct ast_frame *f=NULL;
 	int res;