BUG/MINOR: quic: handle Tx packet allocation failure properly

BUG/MINOR: quic: handle Tx packet allocation failure properly

qc_prep_app_pkts() is responsible to built several new packets for
sending. It can fail due to memory allocation error. Before this patch,
the Tx buffer was released on error even if some packets were properly
generated.

With this patch, if an error happens on qc_prep_app_pkts(), we still try
to send already built packets if Tx buffer is not empty. The sending
loop is then interrupted and the Tx buffer is released with data
cleared.

This should be backported up to 2.7.