[image] Avoid claiming zero-length images as valid

Both the script and PXE images types will claim a zero-length image.
Inhibit this to avoid end-user surprises.

src/arch/i386/image/pxe_image.c

 	if ( filesz > ( 0xa0000 - 0x7c00 ) )
 		return -ENOEXEC;
 
+	/* Rejecting zero-length images is also useful, since these
+	 * end up looking to the user like bugs in gPXE.
+	 */
+	if ( ! filesz )
+		return -ENOEXEC;
+
 	/* There are no signature checks for PXE; we will accept anything */
 	if ( ! image->type )
 		image->type = &pxe_image_type;

src/image/script.c

 	static const char magic[] = "#!gpxe\n";
 	char test[ sizeof ( magic ) - 1 ];
 
+	/* Sanity check */
+	if ( image->len < sizeof ( test ) ) {
+		DBG ( "Too short to be a script\n" );
+		return -ENOEXEC;
+	}
+
 	/* Check for magic signature */
 	copy_from_user ( test, image->data, 0, sizeof ( test ) );
 	if ( memcmp ( test, magic, sizeof ( test ) ) != 0 ) {