Use zero if GIF background color index is missing #5390
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Resolves #4943
For the broken image from that issue, the Global Color Table Flag isn't set. Without it, https://www.w3.org/Graphics/GIF/spec-gif89a.txt
And yet, the disposal method is 2, 'Restore to background color'. So, we need to restore to the background color without knowing what that background color is.
I feel like this is a gap in the specification. If we treat the missing background color as zero though, the image loads correctly. That seems like a better option than our current behaviour of catching the error and just continuing on with the last good disposal that we had.
I presume this missing background color was the
KeyErrorbeing caught, so I've removed that.