[libpng16] fix: Correct the function `png_fp_sub` in png.c

The code erroneously evaluated `addend0+addend1` in the case where
`addend1` is less than zero.  The function is meant to subtract the
second argument from the first.

This is a cherry-pick of commit 79fd6d1edc8fe8c41ed58c6318bd57761d8f007e
from branch 'libpng18'.

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>
1 file changed