lib/genl: avoid VLA in cmd_msg_parser()
lib/genl: avoid VLA in cmd_msg_parser()

We want to build with -Wvla, because VLAs interfere with static asserts
(if the condition of a static assert is not actually static, then VLAs
make it silently pass).

Also, VLAs should be avoided because we want to be in contol how much
we allocate on the stack.

thom311/libnl,thom311/libnl,thom311/libnl,greearb/libnl-ct,greearb/libnl-ct,greearb/libnl-ct