fix #350 panic on receiverQueueSize set to -1 (#361)

Fixes #350

### Verifying this change

- [x] Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

### Does this pull request potentially affect one of the following parts:

  - Dependencies (does it add or upgrade a dependency): no
  - The public API: no
  - The schema: no
  - The default values of configurations: no
  - The wire protocol: no

### Documentation

  - Does this pull request introduce a new feature? no
diff --git a/pulsar/consumer_impl.go b/pulsar/consumer_impl.go
index 97afbaa..13339af 100644
--- a/pulsar/consumer_impl.go
+++ b/pulsar/consumer_impl.go
@@ -93,7 +93,7 @@
 	}
 
 	if options.ReceiverQueueSize <= 0 {
-		options.ReceiverQueueSize = 1000
+		options.ReceiverQueueSize = defaultReceiverQueueSize
 	}
 
 	if options.Interceptors == nil {
diff --git a/pulsar/reader_impl.go b/pulsar/reader_impl.go
index 8083b06..17a7084 100644
--- a/pulsar/reader_impl.go
+++ b/pulsar/reader_impl.go
@@ -85,7 +85,7 @@
 	subscriptionName += "-" + generateRandomName()
 
 	receiverQueueSize := options.ReceiverQueueSize
-	if receiverQueueSize == 0 {
+	if receiverQueueSize <= 0 {
 		receiverQueueSize = defaultReceiverQueueSize
 	}