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).
Safely convert num_bytes to a png_byte in png_set_sig_bytes() (Robert
Seacord).
