mp3dec: Fix possibly exploitable crash

I was sadly unable to find a non fuzzed mp3 that uses the
feature that contained the bug (and i searched hard ...), thus
while this fixes the security issue. It may or may not fix
mixed blocks in 8khz mp3s, i cant say due to lack of samples to test.

Security issue exists since: b37d945d

Reported-by: Dale Curtis <dalecurtis@google.com>
(Probably) Found-by:  <inferno@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>