msm: ADSPRPC: Null check for file session context during device release

While releasing the device, check if the file session context is
NULL to make sure we are not dereferencing during file free.

Change-Id: I4ba08cf9a1d56a5e28e6230b9f2fe02a52b70cb0
Acked-by: Vishnu Karthik D <vikarthi@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>