The fixed size buffer for the file name being processed could have a byte
written beyond the end; a bug where the test was updated without changing the
size of the buffer.  This commit reduces the buffer to the system maximum.

png_getrowbytes could, in theory, return 0; probably only if there is a bug in
libpng but the code now checks.
Signed-off-by: NJohn Bowler <jbowler@acm.org>