Fast bitmap scaling overflowed the LUT used by the nearest-neighbor algorithm.
When a bitmap has 46k pixel on a side and is enlarged, the scaling code
overflows the 32-bit long, resulting in negative indexes, which then segfaults.

This isn't as rare as it sounds. At least in web-view in writer the border/shadow
bitmap is as long as the document (which is an issue in its own right,)
which can overflow for large documents during scaling and segfault.

Change-Id: I1ccf73d02469f6601a9a7e67b30524cb497cf6bc
Reviewed-on: https://gerrit.libreoffice.org/14597Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>