Add default handling for switch statement (#10408)

Motivation:

When a switch statement is used we should always define a `default:` so we don't introduce bugs due fall-through.

Modification:

Add missing `default:`s

Result:

Less error-prone code

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -1055,6 +1055,8 @@ public final class PlatformDependent {
                     case 'g': case 'G':
                         maxDirectMemory *= 1024 * 1024 * 1024;
                         break;
+                    default:
+                        break;
                 }
                 break;
             }

transport/src/main/java/io/netty/channel/nio/NioHandler.java

@@ -625,6 +625,8 @@ public final class NioHandler implements IoHandler {
                     invokeChannelUnregistered(task, k, null);
                 }
                 break;
+            default:
+                 break;
             }
         }
     }