Double free check in malloc.c for 64 bit architectures

[Claude Zou <claude dot zou dot work at gmail dot com>]

Hi,

I was looking at malloc.c, and noticed that SIZE_BITS potentially has
extra space in 64 bit architectures - specifically, the 8 bit.

I wrote up a quick patch that uses that bit to check for double frees
for fastbins more effectively than the current check. Would this be a
feasible addition to glibc? This is my first time doing something like
this, so I apologize if I'm doing something wrong.

Anyways, I've attached a diff with my proposed changes - please let me
know what you think!

Sincerely,
Claude

Attachment: double_free_check.diff
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