msm: ADSPRPC: Fix for NULL pointer dereference

verify fl->sctx to avoid NULL pointer dereference
in fastrpc_device_ioctl

Change-Id: Ie6e45d4e5ddbfb592bf2a891730aab071ffbd538
Acked-by: Chenna Kesava Raju <chennak@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>