diag: Prevent out of bound access while getting build mask

Add check for minimum length before typecasting to build mask
structure to prevent out of bound access.

CRs-Fixed: 2431005
Change-Id: I97b439ead62c8a67869c9209442ef771308f2d3f
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>