AST-2018-002: Crash with an invalid SDP media format description

pjproject's media format parsing algorithm failed to catch invalid values.
Because of this Asterisk would crash if given an SDP with a invalid media
format description.

When parsing the media format description this patch now properly parses the
value and returns an error status if it can't successfully parse/convert the
value.

ASTERISK-27582 #close

Change-Id: I883b3a4ef85b6972397f7b56bf46c5779c55fdd6