avcodec/pngdsp: fix (un)signed type in end comparission

Fixes out of array accesses
Fixes Ticket2919

Found_by: ami_stuff
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
av_cold void ff_pngdsp_init(PNGDSPContext *dsp)
{
    dsp->add_bytes_l2         = add_bytes_l2_c;
    dsp->add_paeth_prediction = ff_add_png_paeth_prediction;

    if (ARCH_X86) ff_pngdsp_init_x86(dsp);
}

av_cold void ff_pngdsp_init(PNGDSPContext *dsp)
{
    dsp->add_bytes_l2         = add_bytes_l2_c;
    dsp->add_paeth_prediction = ff_add_png_paeth_prediction;

    if (ARCH_X86) ff_pngdsp_init_x86(dsp);
}
