Disallow scheduling already modified fibers.

While not strictly needed for correctness, this discourages misuse of
fibers and mixing both coroutine, error handling, and scheduling
purposes.

src/core/ev.c

@@ -3038,6 +3038,9 @@ JANET_CORE_FN(cfun_ev_go,
         fiber->env->proto = janet_vm.fiber->env;
     } else {
         fiber = janet_getfiber(argv, 0);
+        if (janet_fiber_status(fiber) != JANET_STATUS_NEW) {
+            janet_panic("can only schedule new fibers where (= (fiber/status f) :new)");
+        }
     }
     fiber->supervisor_channel = supervisor;
     janet_schedule(fiber, value);

test/suite-ev2.janet

@@ -78,4 +78,10 @@
 (ev/sleep 0.1) # can we do better?
 (assert (= 0 (ev/count supervisor)) "empty supervisor")
 
+# Issue #1707
+(def f (coro (repeat 10 (yield 1))))
+(resume f)
+(assert-error "cannot schedule non-new fiber"
+              (ev/go f))
+
 (end-suite)