tiffdec: Use the correct height field.

Fixes Ticket913
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiff.c

@@ -772,7 +772,7 @@ static int tiff_decode_tag(TiffContext *s)
         break;
     case TIFF_ROWSPERSTRIP:
         if (type == TIFF_LONG && value == UINT_MAX)
-            value = s->avctx->height;
+            value = s->height;
         if (value < 1) {
             av_log(s->avctx, AV_LOG_ERROR,
                    "Incorrect value of rows per strip\n");