[bzimage] Fix spurious uninitialised-variable warning on some gcc versions

Reported-by: Matthew Helton <mwhelton@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/bzimage.c

 		offset = ( ( sizeof ( cpio ) + name_len + 0x03 ) & ~0x03 );
 	} else {
 		offset = 0;
+		name_len = 0;
 	}
 
 	/* Copy in initrd image body (and cpio header if applicable) */