tiffenc: initialize return value

'ret' can only be used without initialization if s->height <= 0, which can
only happen if avctx->height <= 0, which is validated elsewhere. Doesn't hurt
to still initialize it though.

CC: libav-stable@libav.org
Bug-Id: CID 732296

libavcodec/tiffenc.c

@@ -214,7 +214,7 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     int bytes_per_row;
     uint32_t res[2]    = { 72, 1 };     // image resolution (72/1)
     uint16_t bpp_tab[] = { 8, 8, 8, 8 };
-    int ret;
+    int ret = 0;
     int is_yuv = 0;
     uint8_t *yuv_line = NULL;
     int shift_h, shift_v;