Only check the first value of the first Via header

diff -r 8fd95ab3ddf0 channels/chan_sip.c
--- a/channels/chan_sip.c	Wed Dec 06 13:10:33 2006 +0100
+++ b/channels/chan_sip.c	Wed Dec 06 14:06:16 2006 +0100
@@ -8894,7 +8894,11 @@ static void check_via(struct sip_pvt *p,
 	struct hostent *hp;
 	struct ast_hostent ahp;
 
+	/* Work on the leftmost value of the topmost Via header */
 	ast_copy_string(via, get_header(req, "Via"), sizeof(via));
+	c = strchr(via, ',');
+	if (c)
+		*c = '\0';
 
 	/* Check for rport */
 	c = strstr(via, ";rport");