CVE-2020-1917

xbuf_format_converter, used as part of exif_read_data, was appending a
terminating null character to the generated string, but was not using its
standard append char function. As a result, if the buffer was full, it
would result in an out-of-bounds write. This issue affects HHVM versions
prior to 4.56.3, all versions between 4.57.0 and 4.80.1, all versions
between 4.81.0 and 4.93.1, and versions 4.94.0, 4.95.0, 4.96.0, 4.97.0,
4.98.0.