ffmdec: initialize f_cprv, f_stvi and f_stau

They are used in a switch statement, but it is not guaranteed that the
COMM case (where they are set to 0) is reached before the other cases.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/ffmdec.c

@@ -261,7 +261,7 @@ static int ffm2_read_header(AVFormatContext *s)
     AVIOContext *pb = s->pb;
     AVCodecContext *codec;
     int ret;
-    int f_main = 0, f_cprv, f_stvi, f_stau;
+    int f_main = 0, f_cprv = -1, f_stvi = -1, f_stau = -1;
     AVCodec *enc;
     char *buffer;