ASoC: msm8974: Fix LPA crash during pause and next clip play

Fix LPA crash during pause and next clip play. In repeat-only
mode, sometimes session free comes first before the callback for
memory unmap regions comes. Added check in the callback to avoid
null pointer access and exit safely.

Change-Id: I42a1bd7e43086b7c2d24e867f7488de26927090c
CRs-Fixed: 403763
Signed-off-by: Phani Kumar Uppalapati <phanik@codeaurora.org>