clang compiler warnings: Fix non-literal-null-conversion warnings

clang compiler warnings: Fix non-literal-null-conversion warnings

Clang will flag errors when a char pointer is set to '\0', as opposed to a
value that the char pointer points to. This patch fixes this warning
in a variety of locations.

Review: https://reviewboard.asterisk.org/r/4551

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4551.patch submitted by dkdegroot (License 6600)
........

Merged revisions 434187 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434188 65c4cc65-6c06-0410-ace0-fbb531ad65f3