BUG/MINOR: h3: fix checking on NULL Tx buffer

BUG/MINOR: h3: fix checking on NULL Tx buffer

The following patch was backported to handle gracefully Tx buffer
allocation failure.

  BUG/MINOR: h3: close connection on sending alloc errors

However, the backport is not correct. Indeed, mux_get_buf() returns a
pointer to <qcs.tx.buf> field which is always true. Instead, an explicit
check using b_is_null() must be done instead.

This must be backported up to 2.6.