Fix many issues from the NULL_RETURNS Coverity report

Fix many issues from the NULL_RETURNS Coverity report

Most of the changes here are trivial NULL checks.  There are a couple
optimizations to remove the need to check for NULL and outboundproxy parsing
in chan_sip.c was rewritten to avoid use of strtok.  Additionally, a bug was
found and fixed with the parsing of outboundproxy when "outboundproxy=," was
set.

(Closes issue ASTERISK-19654)
........

Merged revisions 365398 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@365399 65c4cc65-6c06-0410-ace0-fbb531ad65f3