sip optimization: avoid calling tvb_get_guint8.

Use tvb_pbrk_guint8, tvb_find_guint8 when possible.

Change-Id: If8090d9b9b92146e9c216f139c056130d6b04e78
Reviewed-on: https://code.wireshark.org/review/2569
Reviewed-by: Anders Broman <a.broman58@gmail.com>