lang/php73: update to 7.3.15

Update php73 to 7.3.15 (PHP 7.3.15).

20 Feb 2020, PHP 7.3.15

- Core:
  . Fixed bug #71876 (Memory corruption htmlspecialchars(): charset `*' not
    supported). (Nikita)
  . Fixed bug ##79146 (cscript can fail to run on some systems). (clarodeus)
  . Fixed bug #78323 (Code 0 is returned on invalid options). (Ivan Mikheykin)
  . Fixed bug #76047 (Use-after-free when accessing already destructed
    backtrace arguments). (Nikita)

- CURL:
  . Fixed bug #79078 (Hypothetical use-after-free in curl_multi_add_handle()).
    (cmb)

-Intl:
  . Fixed bug #79212 (NumberFormatter::format() may detect wrong type). (cmb)

- Libxml:
  . Fixed bug #79191 (Error in SoapClient ctor disables DOMDocument::save()).
    (Nikita, cmb)

- MBString:
  . Fixed bug #79154 (mb_convert_encoding() can modify $from_encoding). (cmb)

- MySQLnd:
  . Fixed bug #79084 (mysqlnd may fetch wrong column indexes with MYSQLI_BOTH).
    (cmb)

- OpenSSL:
  . Fixed bug #79145 (openssl memory leak). (cmb, Nikita)

- Phar:
  . Fixed bug #79082 (Files added to tar with Phar::buildFromIterator have
    all-access permissions). (CVE-2020-7063) (stas)
  . Fixed bug #79171 (heap-buffer-overflow in phar_extract_file).
    (CVE- 2020-7061) (cmb)
  . Fixed bug #76584 (PharFileInfo::decompress not working). (cmb)

- Reflection:
  . Fixed bug #79115 (ReflectionClass::isCloneable call reflected class
    __destruct). (Nikita)

- Session:
  . Fixed bug #79221 (Null Pointer Dereference in PHP Session Upload Progress).
    (CVE-2020-7062) (stas)

- SPL:
  . Fixed bug #79151 (heap use after free caused by
    spl_dllist_it_helper_move_forward). (Nikita)

- Standard:
  . Fixed bug #78902 (Memory leak when using stream_filter_append). (liudaixiao)

- Testing:
  . Fixed bug #78090 (bug45161.phpt takes forever to finish). (cmb)

- XSL:
  . Fixed bug #70078 (XSL callbacks with nodes as parameter leak memory). (cmb)

(taca)