Returns the number of bytes remaining in the input buffer; normally
called when finished() is true to determine amount of post-gzip-stream
data. Note that, other than the finished state with concatenated data
after the end of the current gzip stream, this will never return a
non-zero value unless called after
#setInput(byte[] b,int off,int len) and before
#decompress(byte[] b,int off,int len).
(That is, after
#decompress(byte[] b,int off,int len) it
always returns zero, except in finished state with concatenated data.)