Fix first row init on read.

The code failed to zero out the row buffer at the start of a pass where the AVG
filter is used because 'row_number' is not always zero (on interlaced images)
and, apparently, the assertion that the buffer had been zeroed when allocated
was not always correct for non-interlaced images.  Revealed by pngvalid --size
with a hack to make it always use AVG.

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

pngwutil.c

@@ -2284,18 +2284,10 @@ png_write_filter_row(png_structrp png_ptr, png_bytep prev_pixels,
             filters_to_try |= PNG_FILTER_SUB/*equialent to PAETH here*/;
          }
 
-#if 0
          /* If this leaves the AVG filter it will be used on the first row
           * this is handled in the filter implementation by setting prev_row
           * to NULL below.
           */
-#else /* DOESN'T WORK (problems in the read code?) */
-         if ((filters_to_try & PNG_FILTER_AVG) != 0U)
-         {
-            filters_to_try &= PNG_BIC_MASK(PNG_FILTER_AVG);
-            filters_to_try |= PNG_FILTER_NONE;
-         }
-#endif
       }
 
       /* Check for a narrow image; the blocking will never return just one