mgmt/mcumgr/lib: Fix no check on message

The commit fixes smp_process_request_packet not checking header
stated message length against actual message length.
This could lead to an erratic behavior of an application when it tries
to parse CBOR data beyond actual size of message or potential
Zephyr crash.
When application has asserts enabled the error leads to inevitable
Zephyr crash, caused by net_buf checks.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>