BUG/MEDIUM: mworker: don't free the wrong child when not found

BUG/MEDIUM: mworker: don't free the wrong child when not found

A bug occurs when the sigchld handler is called and a child which is
not in the process list just left, or with an empty process list.

The child variable won't be set and left as an uninitialized variable or
set to the wrong child entry, which can lead to a free of this
uninitialized variable or of the wrong child.

This can lead to a crash of the master during a stop or a reload.

It is not supposed to happen with a worker which was created by the
master. A cause could be a fork made by a dependency. (openssl, lua ?)

This patch strengthens the case of the missing child by doing the free
only if the child was found.

This patch must be backported to 1.9.