sandbox: Fix up the debug message for the image filename

This currently prints out the wrong filename. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/os.c b/arch/sandbox/cpu/os.c
index 62e05c5..a8d01e4 100644
--- a/arch/sandbox/cpu/os.c
+++ b/arch/sandbox/cpu/os.c
@@ -668,7 +668,7 @@
 	os_free(argv);
 	if (err) {
 		perror("Unable to run image");
-		printf("Image filename '%s'\n", mem_fname);
+		printf("Image filename '%s'\n", fname);
 		return err;
 	}