Don't throw warnings during heredoc scan-ahead

Otherwise these warnings will turn up twice (or more...)

Author: nikic

Zend/tests/warning_during_heredoc_scan_ahead.phpt

@@ -0,0 +1,20 @@
+--TEST--
+No warnings should be thrown during heredoc scan-ahead
+--FILE--
+<?php
+
+<<<TEST
+${x
}
+\400
+${/*}
+TEST;
+
+?>
+--EXPECTF--
+Warning: Unexpected character in input:  '
' (ASCII=1) state=0 in %s on line %d
+
+Warning: Octal escape sequence overflow \400 is greater than \377 in %s on line %d
+
+Warning: Unterminated comment starting line %d in %s on line %d
+
+Parse error: syntax error, unexpected end of file in %s on line %d