WPS NFC: Fix potential NULL pointer dereference on an error path

WPS NFC: Fix potential NULL pointer dereference on an error path

The NFC connection handover specific case of WPS public key generation
did not verify whether the two wpabuf_dup() calls succeed. Those may
return NULL due to an allocation failure and that would result in a NULL
pointer dereference in dh5_init_fixed().

Fix this by checking memory allocation results explicitly. If either of
the allocations fail, do not try to initialize wps->dh_ctx and instead,
report the failure through the existing error case handler below.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org