pngdec: Stop trying to decode once inflate returns Z_STREAM_END

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>
(cherry picked from commit a81cad8f86d1feb7e4bfae29e43f3e994935a5c7)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit a63e83cd4b43c3dcef38f7fefe41c002a263af0f)