msm: vidc: avoid OOB write while accessing memory

Exclude 4 bytes which holds the size of the buffer while calculating
the actual buffer size to avoid OOB write.

Change-Id: I965e78097a3680065aaf7609d35af1a42fc44824
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>