remoteproc: xlnx: Only access buffer information if IPI is buffered

remoteproc: xlnx: Only access buffer information if IPI is buffered

In the receive callback check if message is NULL to prevent
possibility of crash by NULL pointer dereferencing.

Signed-off-by: Ben Levinsky <ben.levinsky@amd.com>
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Fixes: 5dfb28c257b7 ("remoteproc: xilinx: Add mailbox channels for rpmsg")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20260303235127.2317955-3-tanmay.shah@amd.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>