hphp: avoid potential buffer read-overrun

Summary: * hphp/runtime/ext_zend_compat/php-src/main/network.cpp:
When the source string has length >= bufsize, php_socket_strerror (with
a non-NULL buffer) would fail to NUL-terminate the result buffer. Write
an explicit NUL byte to ensure that.  Currently, no caller of this
function exercises the affected code paths, because the "buf"
parameter is always NULL.

Reviewed By: @ptarjan

Differential Revision: D1463545