Bluetooth: Mesh: Network buffer overflow on too long proxy messages

[trond-snekvik]

The network decode function does not check the max length of the input mesage, which can exceed the spec-mandated 29 bytes when coming from the proxy module. This can overflow the decode out buffer, which is defined as 29 bytes in proxy and net_recv.
By default, this will assert in net/buf.c, but if netbuf asserts are disabled, this will overflow the target buffer on stack.

Fixed by #31909.