Fix SSL_pending() and SSL_has_pending() with DTLS

If app data is received before a Finished message in DTLS then we buffer
it to return later. The function SSL_pending() is supposed to tell you
how much processed app data we have already buffered, and SSL_has_pending()
is supposed to tell you if we have any data buffered (whether processed or
not, and whether app data or not).

Neither SSL_pending() or SSL_has_pending() were taking account of this
DTLS specific app data buffer.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18868)