Fix QSvgFeGaussianBlur buffer allocation overflowing

QSvgFeGaussianBlur::apply allocates an array that will used in
gaussian blur calculations. Unlike the usual QImage allocations that
goes through QImageIOHandler, this buffer does not do that. This
change addresses this issue by safely checking the buffer size and
comparing it to the allocation limit read from QImageReader to
prevent size calculations and buffer allocation from overflowing or
exceeding QImageReader::allocationLimit.

Fixes: QTBUG-149766
Pick-to: 6.12 6.11
Change-Id: If025ef6b84646dd311138fef168f0753084c451d
Reviewed-by: Robert Löhning <robert.loehning@qt.io>