client: fix panic when writing packets after connection error (#681)

* Fix writer nullpointer panic on network reconnect

* add additional code and tests

---------

Co-authored-by: aler9 <46489434+aler9@users.noreply.github.com>

async_processor_test.go

@@ -0,0 +1,24 @@
+package gortsplib
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAsyncProcessorStopAfterError(t *testing.T) {
+	p := &asyncProcessor{bufferSize: 8}
+	p.initialize()
+
+	p.push(func() error {
+		return fmt.Errorf("ok")
+	})
+
+	p.start()
+
+	<-p.stopped
+	require.EqualError(t, p.stopError, "ok")
+
+	p.stop()
+}