sdm660: ADSPRPC: Null check file session ctx during mmap create

During fastrpc mmap create and mmap free, we are dereferencing
file session context without checking for Null.

Change-Id: I65fc2bb566e2e6e7914b86044e17577a96db6b4f
Acked-by: Vishnu Karthik D <vikarthi@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>