malloc: Add size check when moving fastbin->tcache

malloc: Add size check when moving fastbin->tcache

By overwriting a forward link in a fastbin chunk that is subsequently
moved into the tcache, it's possible to get malloc to return an
arbitrary address [0].

When a chunk is fetched from a fastbin, its size is checked against the
expected chunk size for that fastbin (see malloc.c:3991). This patch
adds a similar check for chunks being moved from a fastbin to tcache,
which renders obsolete the exploitation technique described above.

Now updated to use __glibc_unlikely instead of __builtin_expect, as
requested.

[0]: https://github.com/shellphish/how2heap/blob/master/glibc_2.39/fastbin_reverse_into_tcache.c

Signed-off-by: Ben Kallus <benjamin.p.kallus.gr@dartmouth.edu>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>