app_dial.c: Moved channel lock to prevent deadlock

app_dial.c: Moved channel lock to prevent deadlock

It's reproducible with pbx_lua, not regular dialplan.

deadlock description:
1. asterisk locks a channel
2. calls function onedigit_goto
3. calls ast_goto_if_exists funciton
4. checks ast_exists_extension -> pbx_extension_helper
5. pbx_extension_helper calls pbx_find_extension
6. Then asterisk starts autoservice in a new thread
7. autoservice run tries to lock the channel again

Because our channel is locked already, autoservice can't lock.
Autoservice can't lock -> autoservice stop is waiting forever.
onedigit_goto waits for autoservice stop.

Resolves: https://github.com/asterisk/asterisk/issues/1335
(cherry picked from commit 4b766d24d078dff701805f18cb41fb59bd6033ce)