Fix "application exiting with {Code:0 Signo:27}" during boot.

2aa9514a06 skips SIGURG, but later code expects
the sigchans array contains consecutive signal numbers.

PiperOrigin-RevId: 300793450

pkg/sentry/sighandling/sighandling.go

@@ -83,12 +83,13 @@ func StartSignalForwarding(handler func(linux.Signal)) func() {
 	// for their handling.
 	var sigchans []chan os.Signal
 	for sig := 1; sig <= numSignals+1; sig++ {
+		sigchan := make(chan os.Signal, 1)
+		sigchans = append(sigchans, sigchan)
+
 		// SIGURG is used by Go's runtime scheduler.
 		if sig == int(linux.SIGURG) {
 			continue
 		}
-		sigchan := make(chan os.Signal, 1)
-		sigchans = append(sigchans, sigchan)
 		signal.Notify(sigchan, syscall.Signal(sig))
 	}
 	// Start up our listener.