fix: out-of-bounds access when bg not in color map

[N-R-K]

Reported by @whowillbellthecat in xyb3rt/sxiv#400

The original issue I encounted was a segfault (occurs approx. 1 in 10 attempts) after accidently opening a particular 1 pixel tracking gif.
As base64: R0lGODlhAQABAID/AMDAwAAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==
Stacktrace:

#0  0x000006e1c8972a68 in img_load_gif (img=0x6e1c8983040, file=0x6e48b17bfe0) at image.c:208
#1  0x000006e1c89731f7 in img_load (img=0x6e1c8983040, file=0x6e48b17bfe0) at image.c:334
#2  0x000006e1c8975ea9 in load_image (new=0) at main.c:304
#3  0x000006e1c8978766 in main (argc=2, argv=0x7f7ffffdfae8) at main.c:939

The offending expression is cmap->Colors[bg].Red (in img_load_gif in image.c). In the example gif, cmap->ColorCount is 2, but bg is set to 255. I don't know if this should occur in a 'valid' gif (maybe with transparency(?), though the example gif looks to have the transparency index set to 0). I ended up patching it locally as an error with:

Index: image.c
--- image.c.orig
+++ image.c
@@ -205,6 +205,10 @@ bool img_load_gif(img_t *img, const fileinfo_t *file)
 
 			ptr = data = (DATA32*) emalloc(sizeof(DATA32) * sw * sh);
 			cmap = gif->Image.ColorMap ? gif->Image.ColorMap : gif->SColorMap;
+			if (bg >= cmap->ColorCount) {
+				err = true;
+				break;
+			}
 			r = cmap->Colors[bg].Red;
 			g = cmap->Colors[bg].Green;
 			b = cmap->Colors[bg].Blue;

Which seems to work okayish (no segfaults).

[XPhyro]

Could not reproduce the issue with the given GIF even after many tens of trials, but the change is sensical and compiles & works normally, so I'll approve.

[eylles]

did tests before and after with multiple gifs specially those with transparency, could not replicate the bug either but the fix is so small to not add it.

[eylles]

rebased in accordance to #34

hope i didn't haste this merge too much 😅

[N-R-K]

The giflib documentation was utter shit. So I had to go look up the source code.

https://github.com/rcancro/giflib/blob/4b0c893cfddf16421bd3f59207fdf65f06e9a10d/lib/gifalloc.c#L55

[eylles]

at this point i'm not surprised giflib has shit tier documentation.