Check if an mp3 header is using a reserved sample rate.

Fixes an invalid read past the end of avpriv_mpa_freq_tab.
Fixes divide-by-zero due to sample_rate being set to 0.

Bug-Id: 705

CC:libav-stable@libav.org
Check mp3 header before calling avpriv_mpegaudio_decode_header().

As indicated in the function documentation, the header MUST be
checked prior to calling it because no consistency check is done
there.

CC:libav-stable@libav.org