libcelt_dec: set sample_fmt.

This fixes a regression introduced by the merging of patch fc2dd2c7.
Signed-off-by: Nicolas George <nicolas.george@normalesup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libcelt_dec.c

@@ -91,6 +91,7 @@ static av_cold int libcelt_dec_init(AVCodecContext *c)
                    "improperly decoded by libcelt for version 0x%x.\n",
                    version, lib_version);
     }
+    c->sample_fmt = AV_SAMPLE_FMT_S16;
     return 0;
 }