Problem with Serial.available() and while(!Serial);

[ffissore]

The following "echo sketch" does not work as expected after #1953

void setup() {
  Serial.begin(9600);

  while (!Serial);
}

void loop() {
  if (Serial.available()) {
    Serial.print('-');
    Serial.print(Serial.read());
    Serial.print('-');
  }
}

It should print the code of each character given as input, sorrounded by two hyphens.
When started, you should see a blank IDE serial monitor. As soon as you type something, like "a", you should see printed
-97--10- (when using new line as line terminator)

Instead serial monitor will immediately display -0-, like I had given the null char as input.

Commenting line while (!Serial); prevents the error from happening.

This has been verified with a yun and with a leonardo

/cc @pbrook @embmicro @cmaglie

[ffissore]

Adding some details: I've used IDE from both ide-1.5.x and ide-1.5.x-avr-toolchain-gcc-4.8.1 branches
By reverting merges 82fe44d and 36331fa regression disappears.

[embmicro]

This can be fixed by initializing peek_buffer to -1 on line 31 in USBAPI.h
int peek_buffer = -1;

#2124

[cmaglie]

Fixed with #2124