avformat/wavdec: Eliminate goto for clang -O0 DCE

Clang is not able to eliminate the reference to ff_spdif_probe() when
there is a goto target in the same block and optimization is disabled.

This fixes the following build failure on OS X:
  ./configure --disable-everything --disable-doc \
    --enable-decoder=pcm_s16le --enable-demuxer=wav \
    --enable-protocol=file --disable-optimizations --cc=clang
  make
  ...
  Undefined symbols for architecture x86_64:
    "_ff_spdif_probe", referenced from:
        _set_spdif in libavformat.a(wavdec.o)
  ld: symbol(s) not found for architecture x86_64
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/wavdec.c

@@ -64,34 +64,30 @@ static void set_spdif(AVFormatContext *s, WAVDemuxContext *wav)
 {
     if (CONFIG_SPDIF_DEMUXER && s->streams[0]->codecpar->codec_tag == 1) {
         enum AVCodecID codec;
-        uint8_t *buf = NULL;
         int len = 1<<16;
         int ret = ffio_ensure_seekback(s->pb, len);
-        int64_t pos = avio_tell(s->pb);
-
-        if (ret < 0)
-            goto end;
 
-        buf = av_malloc(len);
+        if (ret >= 0) {
+            uint8_t *buf = av_malloc(len);
             if (!buf) {
                 ret = AVERROR(ENOMEM);
-            goto end;
-        }
-
+            } else {
+                int64_t pos = avio_tell(s->pb);
                 len = ret = avio_read(s->pb, buf, len);
-        if (ret < 0)
-            goto end;
-
+                if (len >= 0) {
                     ret = ff_spdif_probe(buf, len, &codec);
                     if (ret > AVPROBE_SCORE_EXTENSION) {
                         s->streams[0]->codecpar->codec_id = codec;
                         wav->spdif = 1;
                     }
-end:
+                }
                 avio_seek(s->pb, pos, SEEK_SET);
+                av_free(buf);
+            }
+        }
+
         if (ret < 0)
             av_log(s, AV_LOG_WARNING, "Cannot check for SPDIF\n");
-        av_free(buf);
     }
 }