Fix return of nil alertErrors

Fixes two cases where nil alert errors were being returned rather than
the underlying error. Calling methods on these nil alert errors can lead
to panics.

Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>

conn.go

@@ -615,7 +615,7 @@ func (c *Conn) readAndBuffer(ctx context.Context) error {
 				return e
 			}
 		} else if err != nil {
-			return e
+			return err
 		}
 	}
 	if hasHandshake {
@@ -650,7 +650,7 @@ func (c *Conn) handleQueuedPackets(ctx context.Context) error {
 				return e
 			}
 		} else if err != nil {
-			return e
+			return err
 		}
 	}
 	return nil