asoc: msm-pcm: move null ptr check after lock

Null pointer for runtime is checked before acquiring the lock.
By the time, it acquires lock and tries to access private
data runtime may be removed which can lead to null ptr
dereference.

Change-Id: I86ca89bd7dbd765c040bab2a292f62c2d3164140
Signed-off-by: Vikram Panduranga <vpandura@codeaurora.org>