Prohibit passing empty KeepAlive to scheduleOn

Summary: Doing so is a bug that can take some time to pin down. Fail at the callsite instead to make this easier.

Reviewed By: yfeldblum

Differential Revision: D27829947

fbshipit-source-id: e3628331ffb687718309e130e8e0f2f78c4eb1aa

folly/experimental/coro/AsyncGenerator.h

@@ -190,6 +190,7 @@ class AsyncGeneratorPromise {
   }
 
   void setExecutor(folly::Executor::KeepAlive<> executor) noexcept {
+    DCHECK(executor);
     executor_ = std::move(executor);
   }
 

folly/experimental/coro/Task.h

@@ -564,6 +564,7 @@ class FOLLY_NODISCARD Task {
 
   void setExecutor(folly::Executor::KeepAlive<>&& e) noexcept {
     DCHECK(coro_);
+    DCHECK(e);
     coro_.promise().executor_ = std::move(e);
   }