microblaze: boot.c: Don't try to find NULL file

Previously if no device tree was passed to microblaze_load_kernel() then
qemu_find_file() would try to find a NULL pointer. To avoid this put a
check around qemu_find_file().
Signed-off-by: NAlistair Francis <alistair.francis@xilinx.com>
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>

hw/microblaze/boot.c

@@ -124,7 +124,7 @@ void microblaze_load_kernel(MicroBlazeCPU *cpu, hwaddr ddr_base,
     kernel_cmdline = qemu_opt_get(machine_opts, "append");
     dtb_arg = qemu_opt_get(machine_opts, "dtb");
     /* default to pcbios dtb as passed by machine_init */
-    if (!dtb_arg) {
+    if (!dtb_arg && dtb_filename) {
         filename = qemu_find_file(QEMU_FILE_TYPE_BIOS, dtb_filename);
     }