Fix memory leak during code_ws BSON decoding

See: bdcbc9586c3468432be663d3500e6941e395c590 (PR #520 fix for PHP-896)

Fixes the following Jenkins failure: https://jenkins.10gen.com/job/mongo-php-driver/mongodb_configuration=single_server,mongodb_server=legacy-release,os_arch=linux64,php_language_version=5.3/238/testReport/junit/php-src.tests/no-servers/bug00896_code_ws_phpt___Test_for_PHP_896__Segfault_decoding_BSON_reads_past_buffer_endpoint__code_ws_/