Optimize sip dissector: avoid calling tvb_get_string_enc()

Fetch header value, only when we need to parse it.

Change-Id: I3c170ef8ab03985c8111a1b84ac1afc87bc8b5ca
Reviewed-on: https://code.wireshark.org/review/2767
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>