[libpng17' Restored a line, "c = b;", that was inadvertently deleted from the

PAETH filtering code from libpng-1.7.0beta24/pngrutil.c and reenabled
the pngimage tests.

pngrutil.c

@@ -3807,6 +3807,7 @@ png_read_filter_row_paeth_1byte_pixel(png_row_infop row_info, png_bytep row,
       /* Calculate the current pixel in a, and move the previous row pixel to c
        * for the next time round the loop
        */
+      c = b;
       a += *row;
       *row++ = (png_byte)a;
    }