`SSL_pending()` only checks if the *current* TLS packet has more data.
There might be more data in SSL's buffers.
`SSL_has_pending()` only checks if there is data in buffers, but does
*not* check if there is a complete TLS packet that can be decoded.
If we only check the first, we will poll on socket events without having
processed all data and may stall. If we only check the second, we would
busy loop without SSL_read() ever giving something.
Add the flag `connssl->input_pending` that is set on incoming data in
the BIO receive. Clear the flag when encountering a CURLE_AGAIN on
the filters receive (via SSL_read()) or see an EOF.
Ref: #17596Closes#17601
1cdac95e2e