BUG/MINOR: quic: do not alloc buf count on alloc failure

BUG/MINOR: quic: do not alloc buf count on alloc failure

The total number of buffer per connection for sending is limited by a
configuration value. To ensure this, <stream_buf_count> quic_conn field
is incremented on qc_stream_buf_alloc().

qc_stream_buf_alloc() may fail if the buffer cannot be allocated. In
this case, <stream_buf_count> should not be incremented. To fix this,
simply move increment operation after buffer allocation.

The impact of this bug is low. However, if a connection suffers from
several buffer allocation failure, it may cause the <stream_buf_count>
to be incremented over the limit without being able to go back down.

This must be backported up to 2.6.