bridge.c: Obey BRIDGE_NOANSWER variable to skip answering channel.

bridge.c: Obey BRIDGE_NOANSWER variable to skip answering channel.

If the BRIDGE_NOANSWER variable is set on a channel, it is not supposed
to answer when another channel bridges to it using Bridge(), and this is
checked when ast_bridge_call* is called. However, another path exists
(bridge_exec -> ast_bridge_add_channel) where this variable was not
checked and channels would be answered. We now check the variable there.

Resolves: #401
Resolves: #1364
(cherry picked from commit 3ea3ec6f0aaebe7bc057bab584f11ef0e7171514)