DPP: Fix GAS client error case handling

DPP: Fix GAS client error case handling

The GAS client processing of the response callback for DPP did not
properly check for GAS query success. This could result in trying to
check the Advertisement Protocol information in failure cases where that
information is not available and that would have resulted in
dereferencing a NULL pointer. Fix this by checking the GAS query result
before processing with processing of the response.

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