BUG/MINOR: checks: do not exit tcp-checks from the middle of the loop

BUG/MINOR: checks: do not exit tcp-checks from the middle of the loop

There's a comment above tcpcheck_main() clearly stating that no return
statement should be placed in the middle, still we did have one after
installing the mux. It looks mostly harmless though as it will only
fail to mark the server as being in error in case of allocation failure
or config issue.

This fix should be backported to 2.0 and probably 1.9 as well.