diag: Check command size against the minimum before parsing

Add check for minimum length before typecasting to build mask
structure to prevent out of bound access while processing
get msg mask command.

CRs-Fixed: 2431047
Change-Id: I5b8341f278b0b46359800e43c604c5671261c728
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>