png: disable broken MMX/SIMD code for bpp <= 2

The decoder was producing different results when ASM was disabled.
Based on a long debug session with Kostya.

libavcodec/pngdec.c

@@ -240,7 +240,7 @@ static void png_filter_row(PNGDSPContext *dsp, uint8_t *dst, int filter_type,
             p      = last[i];
             dst[i] = p + src[i];
         }
-        if (bpp > 1 && size > 4) {
+        if (bpp > 2 && size > 4) {
             /* would write off the end of the array if we let it process
              * the last pixel with bpp=3 */
             int w = bpp == 4 ? size : size - 3;