diag: Add checks before allocating and freeing memory

Diag driver uses memory pool for internal  buffers. This patch adds
checks for freeing the buffer in a specific mode and while allocating
buffer in all modes.

Change-Id: Ib04f4d722c5cd27e5ef39a1ae65dc74bfe0d136e
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>