chan_unistim: Unlock mutex in rare OOM condition.

#ASTERISK-23792 #close
Reported by: Peter Whisker

Review: https://reviewboard.asterisk.org/r/3567/
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Merged revisions 414677 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 414678 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@414679 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_unistim.c

@@ -6384,10 +6384,10 @@ static struct unistim_device *build_device(const char *cat, const struct ast_var
 		}
 		d = d->next;
 	}
+	ast_mutex_unlock(&devicelock);
 	if (!(lt = ast_calloc(1, sizeof(*lt)))) {
 		return NULL;
 	}
-	ast_mutex_unlock(&devicelock);
 	if (create) {
 		if (!(d = ast_calloc(1, sizeof(*d)))) {
 			return NULL;