Fix parameter numbers and missing alpha check for imagecolorset()

The check for the alpha parameter existed in PHP 7.4 but was lost in PHP 8.0.

Fixes: 5076507

Author: thg2k

ext/gd/gd.c

@@ -2212,9 +2212,10 @@ PHP_FUNCTION(imagecolorset)
 
 	im = php_gd_libgdimageptr_from_zval_p(IM);
 
-	CHECK_RGBA_RANGE(red, Red, 2);
-	CHECK_RGBA_RANGE(green, Green, 3);
-	CHECK_RGBA_RANGE(blue, Blue, 4);
+	CHECK_RGBA_RANGE(red, Red, 3);
+	CHECK_RGBA_RANGE(green, Green, 4);
+	CHECK_RGBA_RANGE(blue, Blue, 5);
+	CHECK_RGBA_RANGE(alpha, Alpha, 6);
 
 	col = color;
 

ext/gd/tests/imagecolorset_error1.phpt

@@ -0,0 +1,23 @@
+--TEST--
+imagecolorset() parameters errors
+--EXTENSIONS--
+gd
+--FILE--
+<?php
+$im = imagecreate(5, 5);
+
+$c = imagecolorallocatealpha($im, 3, 4, 5, 6);
+
+trycatch_dump(
+    fn() => imagecolorset($im, $c, -3, 4, 5, 6),
+    fn() => imagecolorset($im, $c, 3, -4, 5, 6),
+    fn() => imagecolorset($im, $c, 3, 4, -5, 6),
+    fn() => imagecolorset($im, $c, 3, 4, 5, -6),
+);
+
+?>
+--EXPECT--
+!! [ValueError] imagecolorset(): Argument #3 ($red) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #4 ($green) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #5 ($blue) must be between 0 and 255 (inclusive)
+!! [ValueError] imagecolorset(): Argument #6 ($alpha) must be between 0 and 127 (inclusive)