Correct the number of available call numbers in IAX2.

There is currently an edge case where call number 32768 might be allocated for
a call, even though the IAX2 protocol requires call numbers be only 15 bits.
This resulted in some unpredictable behavior when call number 32678 is chosen.

This patch was mostly written by Richard Mudgett via ReviewBoard.  I'm just
committing it.

Review: https://reviewboard.asterisk.org/r/2293/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@380254 f38db490-d61c-443f-a65b-d21fe96a405b