chunk reading.

png_error() on failure.  Reject oversized iCCP profile immediately.

In libpng 1.6 zlib initialization was changed to use the window size in the zlib
stream, not a fixed value.  This causes some invalid images, ones where CINFO is
too large, to display 'correctly' if the rest of the data is valid.  This
provides a work-round for zlib versions where the error arises (ones that
support the API change to use the window size in the stream).
Signed-off-by: NJohn Bowler <jbowler@acm.org>

png_handle_pCAL() (Bug report by John Regehr).

replacing the value that is illegal in the PNG spec, in both signed and
unsigned values, with 0. Illegal unsigned values (anything greater than or equal
to  0x80000000) can still pass through, but since these are not illegal
in ANSI-C (unlike 0x80000000 in the signed case) the checking that
occurs later can catch them (John Bowler).

(Bug report by Viktor Szaka'ts).  Several warnings remain and are
unavoidable, where we test for overflow.