drivers/nble: Fix NULL pointer dereference

This fixes conn->gatt_private = NULL pointer dereference that occurs
in the next Read Blob Response, here:

/* TODO: Get params from user_data pointer, not working at the moment */
params = conn->gatt_private;

Change-Id: Icf280c856133a4c91fc10475d3a047809f917f46
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>