asyncChannel: fixes potential crashes when no more awaiting (#143)

This commit fixes crashes where the state is "awaiting" with no more awaiting clients.
The state is now set to .idle to reflect the reality.

Author: twittemb

Sources/AsyncAlgorithms/AsyncChannel.swift

@@ -89,7 +89,11 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
       switch self {
       case .awaiting(var awaiting):
         let continuation = awaiting.remove(Awaiting(placeholder: generation))?.continuation
-        self = .awaiting(awaiting)
+        if awaiting.isEmpty {
+          self = .idle
+        } else {
+          self = .awaiting(awaiting)
+        }
         return continuation
       case .idle:
         self = .awaiting([Awaiting(cancelled: generation)])
@@ -145,7 +149,11 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
             nexts.remove(Awaiting(placeholder: generation))
             cancelled = true
           }
-          state.emission = .awaiting(nexts)
+          if nexts.isEmpty {
+            state.emission = .idle
+          } else {
+            state.emission = .awaiting(nexts)
+          }
           return nil
         }
       }?.resume()