diag: Do not include unallocated buffer to md table

Currently there is a possibility of copying apps data
buffer to diag_md table after getting freed. The patch
adds buffer to table only when buffer is allocated.

Change-Id: Ie78a74f64cb3c3e3a49077f8ed33b3f2662ea29e
Signed-off-by: Hardik Arya <harya@codeaurora.org>