#32 Fix of spoilt image counter

[Kobrin Eli]

Hi! I've found an error in gif library. The problem is with the ImageCount counter of read images. In functionDGifSlurp when we call the DGetImageDesc function in case it returns OK, the image counter is increased. Later, if the sizes of image are incorrect, the error will be returned, RasterBits buffer won't be allocated, but image counter will stay the same. That may lead to delusion that the image was actually read, and as a result to null pointer dereference (see https://github.com/tensorflow/tensorflow/pull/61913).

To prevent this, I've added the decreasing of image counter in error case with corresponding freeing of spoilt image.

[Kobrin Eli]

Here is the fixed patch

[Kobrin Eli]

Better use this

[Eric S. Raymond]

Applied, thanks.