usb: dwc3: Avoid NULL pointer access

During Halt Endpoint CH9 test with RNDIS composition
NULL pointer dereference is occurring in
__dwc3_gadget_ep_set_halt function which is trying to
access trb_pool which is already freed. In this case
set_alt 0 is getting called before__dwc3_gadget_ep_set_halt.
As a part of set_alt, ipa_disconnect_handler is getting
called which is freeing the trbs. __dwc3_gadget_ep_set_halt
function is called as a part of std_request from Host side
to halt endpoint. This change adds a NULL check before accessing
trb_pool.

Change-Id: Id51494310f713b9c593e29449442661a1170ee6f
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>