[patch v5] malloc: avoid need for tcache == NULL checks

[Wilco Dijkstra]

Hi DJ,

>    Avoid needing to check for tcache == NULL by initializing it
>    to a dummy read-only tcache structure.  This dummy is all zeros,
>    so logically it is both full (when you want to put) and empty (when
>    you want to get).  Also, there are two dummies, one used for
>    "not yet initialized" and one for "tunables say we shouldn't have
>    a tcache".

LGTM.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Cheers,
Wilco