libavformat/tls_libtls: handle TLS_WANT_{POLLIN, POLLOUT} in read/write functions

According to the tls documentation: tls_read() and tls_write() can
return TLS_WANT_POLLIN and TLS_WANT_POLLOUT which indicates that the
same operation must be repeated immediately.

This commit prevents the libtls backend from failing when libtls returns
TLS_WANT_POLLIN or TLS_WANT_POLLOUT with the following error:

    [tls @ 0x7f6e20005a00] (null)
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/tls_libtls.c

@@ -158,6 +158,8 @@ static int ff_tls_read(URLContext *h, uint8_t *buf, int size)
         return ret;
     else if (ret == 0)
         return AVERROR_EOF;
+    else if (ret == TLS_WANT_POLLIN || ret == TLS_WANT_POLLOUT)
+        return AVERROR(EAGAIN);
     av_log(h, AV_LOG_ERROR, "%s\n", tls_error(p->ctx));
     return AVERROR(EIO);
 }
@@ -171,6 +173,8 @@ static int ff_tls_write(URLContext *h, const uint8_t *buf, int size)
         return ret;
     else if (ret == 0)
         return AVERROR_EOF;
+    else if (ret == TLS_WANT_POLLIN || ret == TLS_WANT_POLLOUT)
+        return AVERROR(EAGAIN);
     av_log(h, AV_LOG_ERROR, "%s\n", tls_error(p->ctx));
     return AVERROR(EIO);
 }