sandbox: Check the filename in jump_to_image_no_args()

If the filename is NULL this function currently crashes. Update it to fail
gracefully.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 5005ed2..2ca4cd6 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -69,7 +69,11 @@
 {
 	const char *fname = spl_image->arg;
 
-	os_fd_restore();
-	os_spl_to_uboot(fname);
+	if (fname) {
+		os_fd_restore();
+		os_spl_to_uboot(fname);
+	} else {
+		printf("No filename provided for U-Boot\n");
+	}
 	hang();
 }