Exit without asserting when helper process startup fails (#1543)

Exit without asserting when helper process startup fails (#1543)

... to dup() after fork() and before execvp().

Assertions are for handling program logic errors. Helper initialization
code already handled system call errors correctly (i.e. by exiting the
newly created helper process with an error), except for a couple of
assert()s that could be triggered by dup(2) failures.

This bug was discovered and detailed by Joshua Rogers at
https://megamansec.github.io/Squid-Security-Audit/ipc-assert.html
where it was filed as 'Assertion in Squid "Helper" Process Creator'.