Fix potential crash from race condition due to accessing channel data without the...

Fix potential crash from race condition due to accessing channel data without the channel locked.

In res_musiconhold.c, there are several places where a channel's
stream's existence is checked prior to calling ast_closestream on it. The issue
here is that in several cases, the channel was not locked while checking the
stream. The result was that if two threads checked the state of the channel's
stream at approximately the same time, then there could be a situation where
both threads attempt to call ast_closestream on the channel's stream. The result
here is that the refcount for the stream would go below 0, resulting in a crash.

I have added proper channel locking to res_musiconhold.c to ensure that
we do not try to check chan->stream without the channel locked. A Digium customer
has been using this patch for several weeks and has not had any crashes since
applying the patch.

ABE-2147

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@260345 65c4cc65-6c06-0410-ace0-fbb531ad65f3