Use the correct buffer size for the Pushback stream.

The maximum number of bytes we can push back to the
stream is the actual size of the DeflaterInputStream
buffer (buf.length) and not the default size (BUF_LENGTH).

bug: https://code.google.com/p/android/issues/detail?id=66409
Change-Id: I78a80b6ced8606602361abd8a969fa6f8227596c