Fix loading SVG table with 0 entries

[natehitze]

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Description

Fixes #534.

This changes SvgTable.Load(BigEndianBinaryReader) to return null when the SVG table has 0 entries. Its only caller is SvgTable.Load(FontReader), which can already return null if the table isn't found.

Added font is licensed CC0: https://ggbot.itch.io/home-video-font

Thought Process

I have no experience with fonts but it seemed like a straightforward change.

When running the debugger, I saw BigEndianBinaryReader.ReadBytes was called with a -9 count:

In the caller SvgTable.Load the numEntries value was read as 0.

This leaves minRelOffset and maxEnd at their initial values since the i < numEntries loop has nothing to do. The addition to calculate tableStart wraps ((uint) 10 + uint.MaxValue = 9). Then the integer subtraction calculates a negative byteCount ((int)(0 - 9) = -9).

The easiest approach was to treat an empty table as the table being missing, even though having no entries doesn't appear to be allowed in the spec. This specific font was working in the previous Fonts version and other programs are able to load and render it, so I think it isn't so invalid that it can't work.