Tidy up QPixmapCache to prevent potential segfault

Following reports of segfaults in long-running programs leading to
QPixmapCache corruption, clean up some code smells:

* check pointers before dereferencing

We did this in some places, and a default constructed or moved-from key
could have a nullptr KeyData, so check everywhere.

* don't trunctate qsizetype to int

Still plenty of int APIs left, but no need for ints in internal code.

* don't underflow maxCost to -1 if totalCost was 0
* use ranged-for to iterate over list of keys
* guard any public function that might create the cache with the thread-
test

This avoids that the cache ends up living in the wrong thread.

* don't use reinterpret_cast when static_cast is enough (which is always
from void*)

Since the crash is not reproducible so far, and the reports indicate
that it can only be observed when the program has run for a long time,
there is no test case included. However, this removes some code smells
that might be responsible for data corruption.

Pick-to: 6.2
Task-number: QTBUG-97903
Task-number: QTBUG-91445
Task-number: QTCREATORBUG-26473
Change-Id: Ibdd8963d7dd44caab1468ecc6f81ace761719c69
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

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