Fix DrawGraph.GetAlphaColor/SACT2.SP_GetAMapValue

Properly handle cases where the coordinate is outside of the sprite.
Fixes an issue on the Haru Urare menu sceen.

src/hll/DrawGraph.c

@@ -191,6 +191,12 @@ static void DrawGraph_CopyReverseAMapLR(int dst, int dx, int dy, int src, int sx
 
 static bool DrawGraph_GetAlphaColor(int surface, int x, int y, int *a)
 {
+	struct sact_sprite *sp = sact_get_sprite(surface);
+	// NOTE: System40.exe errors here
+	if (!sp)
+		VM_ERROR("Invalid sprite index: %d", surface);
+	if (x < 0 || x >= sp->rect.w || y < 0 || y >= sp->rect.h)
+		return false;
 	*a = sact_SP_GetAMapValue(surface, x, y);
 	return true;
 }

src/hll/SACT2.c

@@ -568,6 +568,8 @@ int sact_SP_GetAMapValue(int sp_no, int x, int y)
 {
 	struct sact_sprite *sp = sact_get_sprite(sp_no);
 	if (!sp) return 0;
+	if (x < 0 || x >= sp->rect.w || y < 0 || y >= sp->rect.h)
+		return -1;
 	return sprite_get_amap_value(sp, x, y);
 }