Fix #79934: CRLF-only line in heredoc causes parsing error

In `strip_multiline_string_indentation()` (Zend/zend_language_scanner.l)
the case that is supposed to ignore whitespace-only lines now correctly
handles CRLF-only lines.

Author: pevdh

Zend/tests/bug79934.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Bug #79934: CRLF-only line in heredoc causes parsing error
+--FILE--
+<?php
+eval("\$s = <<<HEREDOC\r\n    a\r\n\r\n    b\r\n    HEREDOC;");
+var_dump($s);
+?>
+--EXPECT--
+string(6) "a
+
+b"