micro-optimization: use ProStringList::join(QChar) overload

Change-Id: I25c6205df78da7fbee0ad1b04476f528f376b7a6
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
fix raw data leak in $$sprintf()

it could be only triggered by abusing the function (no expansion
actually done), and nobody is using this to start with, but still ...

Change-Id: I3d4a23ae4d1eea07955572d8213094e0dc218f6d
Reviewed-by: Daniel Teske <daniel.teske@digia.com>