fpga: xilinx: Check valid desc structure

FPGA validation can fail and return value needs to be checked.

Addresses-Coverity-ID: CID 583150: Null pointer dereferences  (NULL_RETURNS)
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/876b6f8dbc99ca305460183dbd18635a35ccc989.1753686468.git.michal.simek@amd.com

drivers/fpga/xilinx.c

@@ -50,6 +50,9 @@ int fpga_loadbitstream(int devnum, char *fpgadata, size_t size,
 	dataptr = (unsigned char *)fpgadata;
 	/* Find out fpga_description */
 	desc = fpga_validate(devnum, dataptr, 0);
+	if (!desc)
+		return FPGA_FAIL;
+
 	/* Assign xilinx device description */
 	xdesc = desc->devdesc;