passt.c: incorrect signal() return value check

In current master, passt.c:main() has an incorrect check on signal()
return value:

               if (sigaction(SIGCHLD, &sa, NULL) || signal(SIGPIPE, SIG_IGN))
                       die("Couldn't install signal handlers");

signal() returns SIG_ERR on failure or the previous setting if present. If
a setting has been inherited from the parent (as is the case when
starting from systemd with the default setting of IgnoreSIGPIPE=yes),
the check will fail.

The if statement should check for SIG_ERR for the SIGPIPE case, or
alternatively switch to sigaction() since that's used for everything
else in the code.

 -Valtteri
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