WPS: Add explicit message length limit of 50000 bytes

WPS: Add explicit message length limit of 50000 bytes

Previously, this was implicitly limited by the 16-bit length field to
65535. This resulted in unhelpful static analyzer warnings (CID 62868).
Add an explicit (but pretty arbitrary) limit of 50000 bytes to avoid
this. The actual WSC messages are significantly shorter in practice, but
there is no specific protocol limit, so 50000 is as good as any limit to
use here.

Signed-off-by: Jouni Malinen <j@w1.fi>