Fix for broken thumbnails when the file width is in $wgThumbnailBucket

When the source file width equals a bucket width,
File::getThumbnailBucket() suggests the use of a thumbnail of that
size. A thumbnailing attempt with the same width as the original
usually returns an error; File::generateBucketsIfNeeded() creates
an empty temporary file to later overwrite with the thumbnail, and
does not clean that file up on error. This empty file is then used
as the intermediary thumbnail for everything in the top bucket.

Bug: T77950
Change-Id: I8c2fdd9b51d227f34cac874ed2b16c87adc9315d