avcodec/pngenc: fix invalid read in sub filter.

First pixel was computed based on invalid address read, and then
corrected by the following memcpy. After the commit, it's not computed
anymore, and memcpy fills the appropriate area.

Fixes Ticket #3387

libavcodec/pngenc.c

@@ -113,6 +113,22 @@ static void sub_png_paeth_prediction(uint8_t *dst, uint8_t *src, uint8_t *top, i
     }
 }
 
+static void sub_left_prediction(DSPContext *dsp, uint8_t *dst, const uint8_t *src, int bpp, int size)
+{
+    const uint8_t *src1 = src + bpp;
+    const uint8_t *src2 = src;
+    int x, unaligned_w;
+
+    memcpy(dst, src, bpp);
+    dst += bpp;
+    size -= bpp;
+    unaligned_w = FFMIN(32 - bpp, size);
+    for (x = 0; x < unaligned_w; x++)
+        *dst++ = *src1++ - *src2++;
+    size -= unaligned_w;
+    dsp->diff_bytes(dst, src1, src2, size);
+}
+
 static void png_filter_row(DSPContext *dsp, uint8_t *dst, int filter_type,
                            uint8_t *src, uint8_t *top, int size, int bpp)
 {
@@ -123,8 +139,7 @@ static void png_filter_row(DSPContext *dsp, uint8_t *dst, int filter_type,
         memcpy(dst, src, size);
         break;
     case PNG_FILTER_VALUE_SUB:
-        dsp->diff_bytes(dst, src, src-bpp, size);
-        memcpy(dst, src, bpp);
+        sub_left_prediction(dsp, dst, src, bpp, size);
         break;
     case PNG_FILTER_VALUE_UP:
         dsp->diff_bytes(dst, src, top, size);