AudioFlinger: protect input/output stream access

Some methods would not check that the output orinput stream of a thread
was still valid before calling functions on its interface.
This could cause a crash if those methods where called while the output or
input was being closed by another thread.

Make sure that the output or input stream pointer is cleared before closing the
stream.
Always check that the output or input pointer is not null before calling
functions at the stream interface.
Generalize the use of initCheck() method to verify that the output or input
stream is not null.

Change-Id: I9d9ca6b744d011bcf3a7bbacb4a581ac1477bfa5