alsa_sound: Fix for unhandled NULL pointer

-Passing NULL pointer to pcm_read()/pcm_write()/
 pcm_prepare() would crash, since it doesnt perform
 a NULL check before using the pointer, resulting
 in random crash
-Fix is to check whether handle is NULL or not.

Change-Id: I7061d40bddcaaae2145f5fc0a0229abeebead878
CRs-fixed: 549257
mm-audio: Fix openmax encoder hang during flush

Flush issued by command thread should unblock the read/write calls
in openmax component.
There is race condition between input/output thread and command
thread in openmax component due to which read/write is not unblocked
which results in a hang.
This change makes command thread to issue multiple flush when it is
wait for ack to avoid a hang.

Change-Id: Ia4486f2e53f56a8413c2115b7f418b29b4bc54c5
CRs-fixed: 466764