JsonReader.peek advances when exception is thrown

When `JsonReader.peek` encounters malformed JSON and throws an exception, it has already modified the state of the reader and advanced one char. This is likely not intended and appears to be the reason why the test `JsonReaderTest.disabled_testNumberWithOctalPrefix` is currently disabled.

Example:
```java
public void testThrowingPeek() throws IOException {
JsonReader reader = new JsonReader(new StringReader("[a?$,1]"));
reader.beginArray();
for (int i = 0; i < 4; i++) {
try {
reader.peek();
} catch (MalformedJsonException expected) {
}
}
reader.nextInt();
reader.endArray();
}
```