AST-2018-003: Crash with an invalid SDP fmtp attribute

pjproject's fmtp retrieval function failed to catch invalid fmtp attributes.
Because of this Asterisk would crash if given an SDP with an invalid fmtp
attribute.

When retrieving the format this patch now makes sure the fmtp attribute is
available. If not available it now returns an error status.

ASTERISK-27583 #close

Change-Id: I5cebe000ce2d846cae3af33b6d72c416e51caf2f