Use the proper lower bound when doing saturation arithmetic.

16 bit signed integers have a range of [-32768, 32768).  The existing code
was using the interval (-32768, 32768) instead.  This patch fixes that.

Review: https://reviewboard.asterisk.org/r/2479/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@386929 f38db490-d61c-443f-a65b-d21fe96a405b