QBuffer: fail early in seek() beyond QByteArray's max capacity

On 32-bit platforms, the range of qsizetype is smaller than the range of the qint64 used as a parameter in seek().

When seek()ing beyond the current buffer's size, the old code relied on a write() to fill the gap with NUL bytes. This has two problems:

First, this may allocate a huge amount of memory just to find that it cannot write that much, possibly even taking the program down when the allocation in the QByteArray ctor fails, instead of returning false from seek().

Second, the QByteArray ctor to which we pass the gapSize only takes qsizetype, not qint64, so we were writing data of size gapSize mod (INT_MAX+1) on 32-bit platforms, which may succeed, just to find that that wasn't the number of bytes we expected to be written. By that time, however, the internal buffer has already been enlarged.

Fix by checking whether the desired seek position is within the limits that QByteArray can contain early on, before attempting to construct such a large QByteArray.

[ChangeLog][QtCore][QBuffer] Fixed silent data corruption on 32-bit platforms when seek() fails due to position > INT_MAX.

Pick-to: 6.3 6.2 5.15 Fixes: QTBUG-102274 Change-Id: Ib63cef7e7e61ef8101a5f056c7b2198bb7baa228 Reviewed-by: Qt CI Bot qt_ci_bot@qt-project.org Reviewed-by: Thiago Macieira thiago.macieira@intel.com (cherry picked from commit 4bc85b98)