chan_dahdi: Fix crash during caller ID read

Asterisk will sometimes core dump during caller id read on analog
channels due to a negative return value from the read() in
my_get_callerid that slips through as a negative length argument to
callerid_feed() if the errno returned by DAHDI is ELAST. This change
ensures that the negative return is treated properly even when it is
ELAST.

(closes issue ASTERISK-22746)
Reported by: Michael Walton
Patches:
    chan_dahdi_cid_crash_fix.r401410.patch uploaded by Michael Walton (License 6502)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@402708 f38db490-d61c-443f-a65b-d21fe96a405b