avcodec/pngdec: Only allow one IHDR chunk

Multiple IHDR chunks are forbidden in PNG
Fixes inconsistency and out of array accesses

Fixes: asan_heap-oob_4d5c5a_1738_cov_2638287726_c-m2-8f2b481b7fd9bd745e620b7c01a18df2.png

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
