avformat/wavdec: Eliminate goto for clang -O0 DCE

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>