Fix segfault in chan_dahdi for CHANNEL(dahdi_span) evaluation on hangup.

Fix segfault in chan_dahdi for CHANNEL(dahdi_span) evaluation on hangup.

* Added NULL private pointer checks in the following chan_dahdi channel
callbacks: dahdi_func_read(), dahdi_func_write(), dahdi_setoption(), and
dahdi_queryoption().

(closes issue ASTERISK-19142)
Reported by: Diego Aguirre
Tested by: rmudgett
........

Merged revisions 349558 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@349559 65c4cc65-6c06-0410-ace0-fbb531ad65f3