chan_misdn segfaults when DEBUG_THREADS is enabled.

chan_misdn segfaults when DEBUG_THREADS is enabled.

The segfault happens because jb->mutexjb is uninitialized from the
ast_malloc().  The internals of ast_mutex_init() were assuming a nonzero
value meant mutex tracking initialization had already happened.  Recent
changes to mutex tracking code to reduce excessive memory consumption
exposed this uninitialized value.

Converted misdn_jb_init() to use ast_calloc() instead of ast_malloc().
Also eliminated redundant zero initialization code in the routine.

(closes issue #18975)
Reported by: irroot

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@312022 65c4cc65-6c06-0410-ace0-fbb531ad65f3