util/cbfstool: Make cbfs_image_delete() NULL-tolerant.

This fixes a double free crash that occurs when a call to
cbfs_image_from_file() fails in cbfs_extract() and falls though to
cbfs_image_delete() with a NULL-pointer.

To reproduce the crash pass the following arguments where the files
passed, in fact, do not exist. As follows:
./cbfstool build/coreboot.rom extract -n config -f /tmp/config.txt

Change-Id: I2213ff175d0703705a0ec10271b30bb26b6f8d0a
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5353
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

util/cbfstool/cbfs_image.c

@@ -300,6 +300,9 @@ int cbfs_image_write_file(struct cbfs_image *image, const char *filename)
 
 int cbfs_image_delete(struct cbfs_image *image)
 {
+	if (image == NULL)
+		return 0;
+
 	buffer_delete(&image->buffer);
 	image->header = NULL;
 	return 0;