Fix GH-16630: UAF in lexer with encoding translation and heredocs

zend_save_lexical_state() can be nested multiple times, for example for
the parser initialization and then in the heredo

Fix GH-16630: UAF in lexer with encoding translation and heredocs

zend_save_lexical_state() can be nested multiple times, for example for
the parser initialization and then in the heredoc lexing. The input
should not be freed if we restore to the same filtered string.

Closes GH-16716.

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