pngdec/filter: dont access out of array elements at the end

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngdec.c

@@ -190,7 +190,7 @@ void ff_add_png_paeth_prediction(uint8_t *dst, uint8_t *src, uint8_t *top, int w
     if(bpp >= 2) g = dst[1];\
     if(bpp >= 3) b = dst[2];\
     if(bpp >= 4) a = dst[3];\
-    for(; i < size; i+=bpp) {\
+    for(; i <= size - bpp; i+=bpp) {\
         dst[i+0] = r = op(r, src[i+0], last[i+0]);\
         if(bpp == 1) continue;\
         dst[i+1] = g = op(g, src[i+1], last[i+1]);\
@@ -206,13 +206,9 @@ void ff_add_png_paeth_prediction(uint8_t *dst, uint8_t *src, uint8_t *top, int w
     else if(bpp == 2) UNROLL1(2, op)\
     else if(bpp == 3) UNROLL1(3, op)\
     else if(bpp == 4) UNROLL1(4, op)\
-    else {\
-        for (; i < size; i += bpp) {\
-            int j;\
-            for (j = 0; j < bpp; j++)\
-                dst[i+j] = op(dst[i+j-bpp], src[i+j], last[i+j]);\
-        }\
-    }
+    for (; i < size; i++) {\
+        dst[i] = op(dst[i-bpp], src[i], last[i]);\
+    }\
 
 /* NOTE: 'dst' can be equal to 'last' */
 static void png_filter_row(PNGDSPContext *dsp, uint8_t *dst, int filter_type,