Fix memory leak in ebur128_init.

Found by Dan Dennedy here:
https://github.com/mltframework/mlt/commit/9d35a6b

Luckily, the impact of the bug is very small, as this code path is not
executed when calling ebur128_init with a non-null mode parameter (which
should be always).
Fix documentation of the EBUR128_MODE_I mode parameter