use new a_crash() asm to optimize double-free handler.

use new a_crash() asm to optimize double-free handler.

gcc generates extremely bad code (7 byte immediate mov) for the old
null pointer write approach. it should be generating something like
"xor %eax,%eax ; mov %al,(%eax)". in any case, using a dedicated
crashing opcode accomplishes the same thing in one byte.