rpmsg: glink: Avoid dereferencing NULL channel

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: a2b73aa512a4 ("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>