Fix GH-17463: SplTempFileObject::ftruncate() segfault on negative length.

Author: devnexen

ext/spl/spl_directory.c

@@ -2706,6 +2706,11 @@ PHP_METHOD(SplFileObject, ftruncate)
 		RETURN_THROWS();
 	}
 
+	if (size < 0) {
+		zend_argument_value_error(1, "must be greater or equal to 0");
+		RETURN_THROWS();
+	}
+
 	CHECK_SPL_FILE_OBJECT_IS_INITIALIZED(intern);
 
 	if (!php_stream_truncate_supported(intern->u.file.stream)) {

ext/spl/tests/gh17463.phpt

@@ -0,0 +1,16 @@
+--TEST--
+GH-15918 (Assertion failure in ext/spl/spl_fixedarray.c)
+--CREDITS--
+YuanchengJiang
+--FILE--
+<?php
+$cls = new SplTempFileObject();
+
+try {
+	$cls->ftruncate(-1);
+} catch (\ValueError $e) {
+	echo $e->getMessage();
+}
+?>
+--EXPECT--
+SplFileObject::ftruncate(): Argument #1 ($size) must be greater or equal to 0