Moc: fix crash when a file ends with \\\r

make the 'cleaned' more robust by making sure we do not read past the
buffer in some cases. We must also use resize and not reserve on the
outpt buffer because reseve is meant as a hint and we are not supposed
to write past the size of the QByteArray even if it is reserved.

[ChangeLog][moc] Fixed crash on file ending with \\\r

Task-number: QTBUG-53441
Change-Id: I901e6c0ffc7f8877de3d07fd08cf26495461d294
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
Reviewed-by: Robert Loehning <robert.loehning@qt.io>