Fix #76041: null pointer access crashed php

We must not draw anti-aliased lines on palette images, because that is
not supported by `gdImageSetAAPixelColor()` and it wouldn't make much
sense to support it, due to the limitation to at most 256 colors.

NEWS

@@ -22,6 +22,7 @@ PHP                                                                        NEWS
   . Fixed ftp_pasv arginfo. (carusogabriel)
 
 -GD:
+  . Fixed bug #76041 (null pointer access crashed php). (cmb)
   . Fixed imagesetinterpolation arginfo. (Gabriel Caruso)
 
 - iconv:

ext/gd/libgd/gd.c

@@ -1239,6 +1239,12 @@ void gdImageAALine (gdImagePtr im, int x1, int y1, int x2, int y2, int col)
 	long x, y, inc, frac;
 	long dx, dy,tmp;
 
+	if (!im->trueColor) {
+		/* TBB: don't crash when the image is of the wrong type */
+		gdImageLine(im, x1, y1, x2, y2, col);
+		return;
+	}
+
 	/* 2.0.10: Nick Atty: clip to edges of drawing rectangle, return if no points need to be drawn */
 	if (!clip_1d(&x1,&y1,&x2,&y2,gdImageSX(im)-1) || !clip_1d(&y1,&x1,&y2,&x2,gdImageSY(im)-1)) {
 		return;

ext/gd/tests/bug76041.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Bug #76041 (null pointer access crashed php)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+$im=imagecreate(100, 100);
+imageantialias($im, true);
+imageline($im, 0, 0, 10, 10, 0xffffff);
+?>
+===DONE===
+--EXPECT--
+===DONE===