pngdec: Stop trying to decode once inflate returns Z_STREAM_END

If the input buffer contains more data after the deflate stream,
the loop previously left running infinitely, with inflate returning
Z_STREAM_END.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>