BUG/MEDIUM: leastconn: fix rare possibility of divide by zero

BUG/MEDIUM: leastconn: fix rare possibility of divide by zero

An optimization was brought in commit 5064ab6a9 ("OPTIM: lb-leastconn:
do not unlink the server if it did not change") to avoid locking the
server just to discover it did not move. However a mistake was made
because the operation involves a divide with a value that is read
outside of its usual lock, which makes it possible to be zero at the
exact moment we watch it if another thread takes the server down under
the lbprm lock, resulting in a divide by zero.

Therefore we must check that the value is not null there.

This must be backported to 2.4.