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
(cherry picked from commit 0562887a98)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/tiffenc.c

@@ -231,7 +231,7 @@ 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;