Enable the silk debug macros with FIXED_DEBUG.

This isn't entirely accurate, since it's also used on integer
math. However, the idea is similar: to add bounds checking.

This code asserts rather than warning, so one must pass
--enable-assertions to configure as well to get any output,
and in that case the first instance is fatal.

Quick testing shows both test_opus_decode and test_opus_encode
trigger the assertions, but running the draft-10 test vectors
does not.