tinyalsa: check for NULL mmap_status

pcm_mmap_get_hw_ptr() was not checking pcm->mmap_status and
crashed when it was NULL.

Bug: 64346189
Test: input_monitor.cpp, see bug report for repro steps
Change-Id: I8436686a231eb01a82dd28ca2cce561d5460bcd6
1 file changed