WPS: Fix use of pre-configured DH keys with multiple operations

WPS: Fix use of pre-configured DH keys with multiple operations

wps_build_public_key() takes the dh_ctx into use and another attempt to
use the same DH keys fails with wps->dh_ctx being set to NULL. Avoid
this by using the DH parameters only if dh_ctx is valid. This fixes
cases where a use of local pre-configured DH keys followed by an
operating using peer DH keys would faild due to unexpected attempt to
use local keys again.

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