gsm0408: Avoid unaligned memory access in gsm48_generate_mid_from_tmsi

The &buf[3] is unlikely to be aligned properly. Use memcpy instead
of an assignment. Add a small testcase that verifies that I didn't
mess up the conversion.

Alignment trap: osmo-nitb (3293) PC=0x492b7094 Instr=0xe5803003 Address=0xbeb259db FSR 0x801
3 files changed