libnsgif and memory bombs

Hi all,

I had a thought regarding memory allocation in libnsgif.

At the moment, calling `gif_initialise()` will parse the GIF file and
allocate memory for rendering. You then call gif_decode_frame() to
decompress and render each frame of the animation.

However, this means just opening the GIF can trigger very large memory
allocations, with no chance for the caller to intervene, except by
putting a hard pixel limit into the bitmap_create() callback. For
example, a 20 byte GIF can cause a 17gb malloc.

How about delaying the allocation of the bitmap until the first call
to gif_decode_frame()? This would allow the caller to check the
detected width and height and apply some kind of size policy before
allowing rendering to occur.

Here's a possible patch that does this. It's very simple and does not
affect the API, just the memory behaviour (I think):

https://github.com/libvips/libvips/commit/9bdf5e8cda3e0c63584984282b1e36d97c50bb1a

John
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