chan_dahdi: fix lower bound check with -ve integer conversion from a float

Lower bound of a 16bit signed int is -32768 not -32767

(closes issue ASTERISK-21744)

Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)
........

Merged revisions 387297 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@387298 65c4cc65-6c06-0410-ace0-fbb531ad65f3