Fixes issue with AST_CONTROL_SRCUPDATE breaking out of native bridge

Fixes issue with AST_CONTROL_SRCUPDATE breaking out of native bridge

In iax2, when a AST_CONTROL_SRCUPDATE is received it breaks from the native bridge, but since there is no code path to handle srcupdate it just goes to be beginning of the loop.  This was causing packet storms of srcupdate frames between servers.  Now srcupdate frames do not break the native bridge for processing.

(closes issue #13749)
Reported by: adiemus

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@176247 65c4cc65-6c06-0410-ace0-fbb531ad65f3