Fix #69024: imagescale segfault with palette based image

imagescale(..., IMG_BICUBIC) is not supposed to work with palette images, so we
fix that by converting to true color if necessary. Basically the same fix has
already been applied to the external libgd[1].

[1] <723ea520be>

ext/gd/libgd/gd_interpolation.c

@@ -1059,6 +1059,11 @@ gdImagePtr gdImageScaleTwoPass(const gdImagePtr src, const unsigned int src_widt
 	gdImagePtr tmp_im;
 	gdImagePtr dst;
 
+	/* Convert to truecolor if it isn't; this code requires it. */
+	if (!src->trueColor) {
+		gdImagePaletteToTrueColor(src);
+	}
+
 	tmp_im = gdImageCreateTrueColor(new_width, src_height);
 	if (tmp_im == NULL) {
 		return NULL;

ext/gd/tests/bug69024.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Bug #69024 (imagescale segfault with palette based image)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+$im = imagecreate(256, 256);
+imagescale($im, 32, 32, IMG_BICUBIC);
+imagedestroy($im);
+echo "done\n";
+?>
+--EXPECT--
+done