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)
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Merged revisions 387297 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 387298 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@387299 65c4cc65-6c06-0410-ace0-fbb531ad65f3