Use nullptr instead of NULL (#1300)

Summary:
Found with modernize-use-nullptr
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Pull Request resolved: https://github.com/facebook/folly/pull/1300

Differential Revision: D19271712

Pulled By: yfeldblum

fbshipit-source-id: e61fe0252a78c0acc3bef57b3d1f152caa2dcd2a

folly/io/async/AsyncSSLSocket.cpp

@@ -1032,7 +1032,7 @@ bool AsyncSSLSocket::willBlock(
 #ifdef SSL_ERROR_WANT_ASYNC
     if (error == SSL_ERROR_WANT_ASYNC) {
       size_t numfds;
-      if (SSL_get_all_async_fds(ssl_.get(), NULL, &numfds) <= 0) {
+      if (SSL_get_all_async_fds(ssl_.get(), nullptr, &numfds) <= 0) {
         VLOG(4) << "SSL_ERROR_WANT_ASYNC but no async FDs set!";
         return false;
       }