Free raw_image if it is unsupported.

If the number of colour components is neither 1 nor 3 we should remember
to free the buffer holding the raw decoded image before returning the
error to Lua.

pic.c

@@ -142,8 +142,10 @@ static int openDocument(lua_State *L) {
 			return luaL_error(L, "Cannot convert to grayscale");
 		else
 			doc->image = gray_image;
-	} else
+	} else {
+		free(raw_image);
 		return luaL_error(L, "Unsupported image format");
+	}
 
 	doc->width = width;
 	doc->height = height;