diag: Prevent accessing uninitialized diag_md_info member

The patch prevents accessing uninitialized diag_md_info
struct with a check on variable set only upon structure
initialization.

Change-Id: Ie0b5ece24fbacfabac876aa4bf2295354374ed57
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>