fix(remember): fix remember() on producer-less streams

remember() method was broken for streams that have no producer, because the generated MemoryStream
would attempt to call producer.start() regardless. This commit fixes that.

src/core.ts

@@ -1598,8 +1598,14 @@ export class Stream<T> implements InternalListener<T> {
    */
   remember(): MemoryStream<T> {
     return new MemoryStream<T>({
-      _start: (il: InternalListener<T>) => { this._prod._start(il); },
-      _stop: () => { this._prod._stop(); },
+      _start: (il: InternalListener<T>) => {
+        const p = this._prod;
+        if (p) p._start(il);
+      },
+      _stop: () => {
+        const p = this._prod;
+        if (p) p._stop();
+      },
     });
   }
 

tests/operator/remember.ts

@@ -102,11 +102,33 @@ describe('Stream.prototype.remember', () => {
     done();
   })
 
+  it('should not fail if original stream has no producer, and start is called', (done) => {
+    const input = xs.create<number>();
+    assert.strictEqual(input instanceof Stream, true);
+    const stream: MemoryStream<number> = input.remember();
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    assert.doesNotThrow(() => {
+      stream.addListener({ next: () => { }, error: () => { }, complete: () => { } });
+    });
+    done();
+  });
+
   it('should return a MemoryStream if input stream is a MemoryStream', (done) => {
     const input = xs.createWithMemory<number>();
     assert.strictEqual(input instanceof MemoryStream, true);
     const stream: MemoryStream<number> = input.remember();
     assert.strictEqual(stream instanceof MemoryStream, true);
     done();
   });
+
+  it('should not fail if original memorystream has no producer, and start is called', (done) => {
+    const input = xs.createWithMemory<number>();
+    assert.strictEqual(input instanceof MemoryStream, true);
+    const stream: MemoryStream<number> = input.remember();
+    assert.strictEqual(stream instanceof MemoryStream, true);
+    assert.doesNotThrow(() => {
+      stream.addListener({ next: () => { }, error: () => { }, complete: () => { } });
+    });
+    done();
+  });
 });