[input_reader] Don't continue if ParseButtons fails

In the event that ParseButtonsReport fails reading buttons, there's
no use trying to continue.  Return early in this case to prevent a
flood of log messages like:

  proxy_controller.cc:54: Tried to decode a bad union discriminant, type
                          fuchsia.ui.input/....
  fdio_hid_decoder.cc(521)] Failed to read from input: -1
  input_interpreter.cc(1008)]  failed reading from buttons

TEST: Built+paved, unplugged usb sound card which has headset buttons
Change-Id: I29d3f8edc6f0effc949bb57e1af708fea162a367
1 file changed