misc: Fix a small pile of warnings found with diagnostics turned way up

* possible null pointer dereferences
 * non-annotated fallthroughs in case statements

Change-Id: I2332e0e0a4203ceca0999291ead112b8ab94c613

apps/plugins/rockboy/mem.c

@@ -269,6 +269,7 @@ static byte ioreg_read(byte r)
     case RI_HDMA4:
     case RI_HDMA5:
         if (hw.cgb) return REG(r);
+        /* Intentional fallthrough */
     default:
         return 0xff;
     }