msm: adsprpc: null pointer check for sctx

Adding a null pointer for sctx before dereferencing.

Change-Id: I1497eea2b9cb7bb75481246df47269796aff182d
Acked-by: Maitreyi Gupta <maitreyi@qti.qualcomm.com>
Signed-off-by: Jeya R <jeyr@codeaurora.org>