BUG/MINOR: mux-h1: Handle read0 in rcv_pipe() only when data receipt was tried

BUG/MINOR: mux-h1: Handle read0 in rcv_pipe() only when data receipt was tried

In rcv_pipe() callback we must be careful to not report the end of stream
too early because some data may still be present in the input buffer. If we
report a EOS here, this will block the subsequent call to rcv_buf() to
process remaining input data. This only happens when we try a last
rcv_pipe() when the xfer length is unknown and all data was already received
in the input buffer. Concretely this happens with a payload larger than a
buffer but lower than 2 buffers.

This patch must be backported as far as 2.7.