Update qstring.cpp

[SpaceAccordeonist]

Set blockSize type to qsizetype.

All variables in evaluation of this value are qsizetype. So it's not a good idea to make it int even if we know that its size is in certain range. Moreover, its size can differ on different platforms.

It can also lead to the problems with overflow a few lines below, where multiplication by 2 takes place.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}