avcodec/lagarith: fix src/src_size for esc_count < 8

untested due to lack of sample
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/lagarith.c

@@ -460,6 +460,8 @@ static int lag_decode_arith_plane(LagarithContext *l, uint8_t *dst,
                    length);
     } else if (esc_count < 8) {
         esc_count -= 4;
+        src ++;
+        src_size --;
         if (esc_count > 0) {
             /* Zero run coding only, no range coding. */
             for (i = 0; i < height; i++) {