moh: fix a refcount error with realtime MOH

I observed a crash in res_musiconhold on an Asterisk 11 system using realtime
MOH.  Investigation of the backtrace showed a corrupt mohclass, implying that
it got destroyed before the code expected it to.  I went looking for reference
counting errors that could have caused this crash and this patch this result.
It contains 2 changes.

1) Remove a usless block of code that was impossible to reach.  There was even
a comment indicating that it was impossible to reach.  The conditional includes
"!ast_test_flag(global_flags, MOH_CACHERTCLASSES)" and it's inside of an if
block with the opposite check "ast_test_flag(global_flags,
MOH_CACHERTCLASSES)".  There's no good reason to keep it around.

2) A similar block to #1 contained a reference counting error.  It stores
state->class in the local variable mohclass without increasing its reference
count.  The reference count on mohclass is decremented at the end of the
function.  This block of code probably very rarely runs, which would help
explain why this system was working fine for many months before experiencing a
crash.

Review: https://reviewboard.asterisk.org/r/3282/
........

Merged revisions 410043 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@410044 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_musiconhold.c

@@ -1471,12 +1471,6 @@ static int local_ast_moh_start(struct ast_channel *chan, const char *mclass, con
 				if (state && state->class) {
 					/* Class already exist for this channel */
 					ast_log(LOG_NOTICE, "This channel already has a MOH class attached (%s)!\n", state->class->name);
-					if (state->class->realtime && !ast_test_flag(global_flags, MOH_CACHERTCLASSES) && !strcasecmp(mohclass->name, state->class->name)) {
-						/* we found RT class with the same name, seems like we should continue playing existing one */
-						/* XXX This code is impossible to reach */
-						mohclass = mohclass_unref(mohclass, "unreffing potential mohclass (channel already has a class)");
-						mohclass = state->class;
-					}
 				}
 				/* We don't want moh_register to unref the mohclass because we do it at the end of this function as well.
 				 * If we allowed moh_register to unref the mohclass,too, then the count would be off by one. The result would
@@ -1529,7 +1523,7 @@ static int local_ast_moh_start(struct ast_channel *chan, const char *mclass, con
 						if (state->class->realtime && !ast_test_flag(global_flags, MOH_CACHERTCLASSES) && !strcasecmp(mohclass->name, state->class->name)) {
 							/* we found RT class with the same name, seems like we should continue playing existing one */
 							mohclass = mohclass_unref(mohclass, "unreffing potential mohclass (channel already has one)");
-							mohclass = state->class;
+							mohclass = mohclass_ref(state->class, "using existing class from state");
 						}
 					} else {
 						if (ast_pthread_create_background(&mohclass->thread, NULL, monmp3thread, mohclass)) {