AST-2017-014: res_pjsip - Missing contact header can cause crash

Those SIP messages that create dialogs require a contact header to be present.
If the contact header was missing from the message it could cause Asterisk to
crash.

This patch checks to make sure SIP messages that create a dialog contain the
contact header. If the message does not and it is required Asterisk now returns
a "400 Missing Contact header" response. Also added NULL checks when retrieving
the contact header that were missing as a "just in case".

ASTERISK-27480 #close

Change-Id: I1810db87683fc637a9e3e1384a746037fec20afe