res_fax: Fix out of bounds error in update_modem_bits().

ASTERISK-24357 #close
Reported by: Jeremy Laine
Patches:
      res_fax_bounds.patch (license #6561) patch uploaded by Jeremy Laine
	  Modified patch to not use magic numbers.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@423979 f38db490-d61c-443f-a65b-d21fe96a405b