BUG/MEDIUM: channel: bad unlikely macro

BUG/MEDIUM: channel: bad unlikely macro

The unlikely macro doesn't take in acount the condition, but only one
variable.

Must be backported in 1.6

[wt: with gcc 3.x, unlikely(x) is defined as __builtin_expect((x) != 0, 0)
 so the condition is wrong for negative numbers, which correspond to the
 case where bi_getblk_nc() has reached the end of the buffer and the
 channel is already closed. With gcc 4.x, the output is cast to unsigned long
 so the <=0 will not match negative values either. This is only used in Lua
 for now so that may explain why it hasn't hit yet]