chan_sip: Allow T.38 switch-over when SRTP is in use.

Previously when SRTP was enabled on a channel it was not possible
to switch to T.38 as no crypto attributes would be present.

This change makes it so it is now possible. If a T.38 re-invite
comes in SRTP is terminated since in practice you can't encrypt
a UDPTL stream. Now... if we were doing T.38 over RTP (which
does exist) then we'd have a chance but almost nobody does that so
here we are.

ASTERISK-24449 #close
Reported by: Andreas Steinmetz
patches:
 udptl-ignore-srtp-v2.patch submitted by Andreas Steinmetz (license 6523)
........

Merged revisions 429632 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/11.6@429857 65c4cc65-6c06-0410-ace0-fbb531ad65f3