rpmsg: glink: Avoid dereferencing NULL channel

The newly introduced signal command handler checks for non-existing
channel and print an error message, but then continues on to dereference
that same channel.

Instead abort the handler when no channel is found.

Fixes: a2b73aa ("rpmsg: glink: Add support to handle signals command")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202307160800.sb7gMnL6-lkp@intel.com/
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Link: https://lore.kernel.org/r/20230717165538.1542034-1-quic_bjorande@quicinc.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Author: quic-bjorande

drivers/rpmsg/qcom_glink_native.c

@@ -1070,8 +1070,10 @@ static void qcom_glink_handle_signals(struct qcom_glink *glink,
 	spin_lock_irqsave(&glink->idr_lock, flags);
 	channel = idr_find(&glink->rcids, rcid);
 	spin_unlock_irqrestore(&glink->idr_lock, flags);
-	if (!channel)
+	if (!channel) {
 		dev_err(glink->dev, "signal for non-existing channel\n");
+		return;
+	}
 
 	enable = sigs & NATIVE_DSR_SIG || sigs & NATIVE_CTS_SIG;