QUIC: fixed chain returned from ngx_quic_write_chain().

QUIC: fixed chain returned from ngx_quic_write_chain().

Previously, when input ended on a QUIC buffer boundary, input chain was not
advanced to the next buffer.  As a result, ngx_quic_write_chain() returned
a chain with an empty buffer instead of NULL.  This broke HTTP write filter,
preventing it from closing the HTTP request and eventually timing out.

Now input chain is always advanced to a buffer that has data, before checking
QUIC buffer boundary condition.