LE: Improved handling of illegal GATT PDUs

Do additional length checks when handling malformed messages. These
messages are never valid, and should be dropped if seen.

Before starting to parse out PDU content, it is verified that the
buffer contains sufficient bytes and error condition is returned
if this is not the case.

Bug: 11045073
Change-Id: Ifb792d1575e0514f33e75ed73247ea638807901e