Fix misuses of asprintf throughout the code.

Fix misuses of asprintf throughout the code.

This fixes three main issues

* Change asprintf() uses to ast_asprintf() so that it
pairs properly with ast_free() and no longer causes
MALLOC_DEBUG to freak out.

* When ast_asprintf() fails, set the pointer NULL if
it will be referenced later.

* Fix some memory leaks that were spotted while taking
care of the first two points.

(Closes issue ASTERISK-20135)
reported by Richard Mudgett

Review: https://reviewboard.asterisk.org/r/2071

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@371590 65c4cc65-6c06-0410-ace0-fbb531ad65f3