Fix out of bounds write im->alpha[im->transparent] (#785)

Since #737 gdImageColorTransparent does not correctly handle the case that im->transparent = -1
(which is the initial value and used to indicate no transparent colour has been set).

This leads to undefined behaviour via an out-of-bound write:
im->alpha[im->transparent] = gdAlphaOpaque;
(in practice I assume this merely overwrites an earlier struct member)

This can be triggered via loading a gif through gdImageCreateFromGifPtr

third_party/gd/source/gd.c:922:2: runtime error: index -1 out of bounds for type 'int [256]'
    #0 0x5629c034a839 in gdImageColorTransparent third_party/gd/source/gd.c:922:29
    #1 0x5629c034ebf0 in gdImageCreateFromGifCtx third_party/gd/source/gd_gif_in.c:328:4
    #2 0x5629c034f14f in gdImageCreateFromGifPtr third_party/gd/source/gd_gif_in.c:186:7

Fixes #784.

src/gd.c

@@ -919,7 +919,9 @@ BGD_DECLARE(void) gdImageColorTransparent (gdImagePtr im, int color)
 	if (color >= gdMaxColors) {
 		return;
 	}
-	im->alpha[im->transparent] = gdAlphaOpaque;
+	if (im->transparent != -1) {
+		im->alpha[im->transparent] = gdAlphaOpaque;
+	}
 	im->alpha[color] = gdAlphaTransparent;
 	im->transparent = color;
 }