Fix #55005: imagepolygon num_points requirement

We actually have to check `$num_points` instead of `2*count($points)`,
because the latter may be greater than the former, but not all elements
of `$points` are guaranteed to be used.  This allowed to pass arrays
with excess elements to draw polygons with less than three vertices.

While the current implementation of `gdImagePolygon()` and friends
would allow us to draw monogons and digons, we don't allow that
anymore, because the respective drawing primitives work slightly
different (e.g. drawing lines support anti-aliasing, but drawing
general polygons does not).

To minimize the BC break, we do not fix this longstanding issue for PHP
7, but target PHP 8 only.

Author: cmb69

NEWS

@@ -17,6 +17,7 @@ PHP                                                                        NEWS
     that allow global flag to configure query() or evaluate() calls.
 
 - GD:
+  . Fixed bug #55005 (imagepolygon num_points requirement). (cmb)
   . Replaced gd resources with objects. (Mark Randall)
   . Removed deprecated image2wbmp(). (cmb)
   . Removed deprecated png2wbmp() and jpeg2wbmp(). (cmb)

ext/gd/gd.c

@@ -2745,13 +2745,8 @@ static void php_imagepolygon(INTERNAL_FUNCTION_PARAMETERS, int filled)
 	col = COL;
 
 	nelem = zend_hash_num_elements(Z_ARRVAL_P(POINTS));
-	if (nelem < 6) {
-		zend_value_error("You must have at least 3 points in your array");
-		return;
-	}
-
-	if (npoints <= 0) {
-		zend_value_error("You must give a positive number of points");
+	if (npoints < 3) {
+		zend_value_error("Polygon must have at least 3 points");
 		return;
 	}
 

ext/gd/tests/bug55005.phpt

@@ -0,0 +1,21 @@
+--TEST--
+Bug #55005 (imagepolygon num_points requirement)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+require_once __DIR__ . '/func.inc';
+
+$g = imagecreate(300, 300);
+$bgnd = imagecolorallocate($g, 255, 255, 255);
+$fgnd = imagecolorallocate($g, 0, 0, 0);
+trycatch_dump(
+    fn () => imagefilledpolygon($g, array(100,10, 100,100, 180,100), 2, $fgnd),
+    fn () => imagepolygon($g, array(200,10, 200,100, 280,100), 2, $fgnd)
+);
+?>
+--EXPECT--
+!! [ValueError] Polygon must have at least 3 points
+!! [ValueError] Polygon must have at least 3 points