bridge_native_rtp.c: Fix reentrancy framehook crash.

If two channels enter different native rtp bridges at the same time it is
possible that the framehook interface data pointer can be corrupted
because the struct variable was declared static.

* Fixed the reentrancy corruption by changing the framehook interface
struct static variable to a stack local variable.

* Moved the hook.data assignment outside of the channel lock.  It did not
need the lock's protection.  It probably was giving a false sense of
security.

The testsuite
channels/pjsip/basic_calls/two_parties/nominal/alice_initiated/bob_hangs_up
test caught this with MALLOC_DEBUG and DO_CRASH enabled.

Change-Id: If9e35b97d19209b0f984941c1d8eb5f7c55eea91