aaudio: free endpoint to prevent crashes

Free the AudioEndpoint and check for nullptr to
prevent accessing shared memory that had been freed.
This is to protect against calls to the stream after
AAudioStream_release() has been called.

Bug: 154274446
Bug: 154274027
Test: libaaudio/tests/test_various.cpp
Change-Id: I194d502fd48c4d31602ffce76aca6b28753ad7d2