bugfix: don't ingore the return value from rts_evalIO()

rts/posix/Signals.c

@@ -148,7 +148,7 @@ ioManagerStart (void)
     Capability *cap;
     if (io_manager_pipe < 0) {
 	cap = rts_lock();
-	rts_evalIO(cap,&base_GHCziConc_ensureIOManagerIsRunning_closure,NULL);
+	cap = rts_evalIO(cap,&base_GHCziConc_ensureIOManagerIsRunning_closure,NULL);
 	rts_unlock(cap);
     }
 }