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)
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Merged revisions 402708 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 402709 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 402710 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402711 65c4cc65-6c06-0410-ace0-fbb531ad65f3