GnuTLS: eat PREMATURE_TERMINATION error

GnuTLS is too strict on the SSL shutdown alert, and it's neither
mandatory in the spec or critical. As it's ignored in OpenSSL, we
should also suppress it in GnuTLS as well.

Ticket: #6667

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/tls_gnutls.c

@@ -72,6 +72,7 @@ static int print_tls_error(URLContext *h, int ret)
     switch (ret) {
     case GNUTLS_E_AGAIN:
     case GNUTLS_E_INTERRUPTED:
+    case GNUTLS_E_PREMATURE_TERMINATION:
         break;
     case GNUTLS_E_WARNING_ALERT_RECEIVED:
         av_log(h, AV_LOG_WARNING, "%s\n", gnutls_strerror(ret));