png_parser: Fix parsing on big endian

Since pc.state is populated by shifting in from the end of the
32 bit word, the content within pc.state is already in native endian
and should not be read with the AV_R{L,B} functions.

This was already done correctly for state64 above.

This fixes the fate-corepng test on big endian.

Signed-off-by: Martin Storsjö <martin@martin.st>