Avoid up-cast warnings in pngvalid.c On ARM the alignment requirements of png_modifier are greater than that of png_store and as a consequence compilation of pngvalid.c results in a warning about increased alignment requirements because of the bare cast to (png_modifier*). The code is safe, because the pointer is known to point to a stack allocated png_modifier, this change avoids the warning.

Signed-off-by: John Bowler <jbowler@acm.org>