vrend: Check to see if a pointer is pointing to a valid buffer before using it in an expression.

Add a helper function to check if a pointer is pointing to a valid buffer before using it in an expression. Use it in vrend_decode_ctx_submit_cmd

Tested: Tested with the fuzzer test that reporter provided with all the crash data. All the heap related issues gone.

Closes:#371

Signed-off-by: Dawn Han dawnhan@google.com