Description:
------------
There is a coding bug in _xmlwriter_get_valid_file_path function in ext/xmlwriter/php_xmlwriter.c. 

>>>
        char file_dirname[MAXPATHLEN];
        size_t dir_len;
        if (!VCWD_REALPATH(source, resolved_path) && !expand_filepath(source, resolved_path)) {
            xmlFreeURI(uri);
            return NULL;
        }
        memcpy(file_dirname, source, strlen(source));
>>>
As code showed above, I think there should be a check for strlen(source) and MAXPATHLEN. If strlen(source) >= MAXPATHLEN, there will be a buffer overflow.

Test script:
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None

Expected result:
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None

Actual result:
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None