usb: musb: trace: fix NULL pointer dereference in musb_g_tx()

The usb_request pointer could be NULL in musb_g_tx(), where the
tracepoint call would trigger the NULL pointer dereference failure when
parsing the members of the usb_request pointer.

Move the tracepoint call to where the usb_request pointer is already
checked to solve the issue.

Fixes: fc78003e5345 ("usb: musb: gadget: add usb-request tracepoints")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_gadget.c

@@ -417,7 +417,6 @@ void musb_g_tx(struct musb *musb, u8 epnum)
 	req = next_request(musb_ep);
 	request = &req->request;
 
-	trace_musb_req_tx(req);
 	csr = musb_readw(epio, MUSB_TXCSR);
 	musb_dbg(musb, "<== %s, txcsr %04x", musb_ep->end_point.name, csr);
 
@@ -456,6 +455,8 @@ void musb_g_tx(struct musb *musb, u8 epnum)
 		u8	is_dma = 0;
 		bool	short_packet = false;
 
+		trace_musb_req_tx(req);
+
 		if (dma && (csr & MUSB_TXCSR_DMAENAB)) {
 			is_dma = 1;
 			csr |= MUSB_TXCSR_P_WZC_BITS;