Fix out-of-bounds read access in GIF decoding

[jserv]

Caught by lldb:

* thread #1, stop reason = EXC_BAD_ACCESS (code=1, address=0x1080d6000) frame #0: 0x00000001000061a4 demo-sdl`_twin_gif_to_pixmap [inlined] gif_is_bgcolor(gif=0x00000001008b4200, color=<unavailable>) at image-gif.c:516:13 [opt] 513 514 	static int gif_is_bgcolor(const twin_gif_t *gif, const uint8_t *color) 515 	{
-> 516 	    return !memcmp(&gif->palette->colors[gif->bgindex * 3], color, 3);
   517 	}
   518
   519 	static void gif_rewind(twin_gif_t *gif)
Target 0: (demo-sdl) stopped.
(lldb) up
frame #1: 0x000000010000619e demo-sdl`_twin_gif_to_pixmap at image-gif.c:584 [opt]
   581 	            uint8_t r = *(color++);
   582 	            uint8_t g = *(color++);
   583 	            uint8_t b = *(color++);
-> 584 	            if (!gif_is_bgcolor(gif, color))
   585 	                *(p.argb32++) = 0xFF000000U | (r << 16) | (g << 8) | b;
   586 	            /* Construct background */
   587 	            else if (((row >> 3) + (col >> 3)) & 1)

The removal of unnecessary pointer arithmetics also help compiler optimizations.