server: Don't crash on blank moods.

server: Don't crash on blank moods.

We special-case empty mood definitions (because we can't map the
osl object anyway), but we don't check whether the mood definition
contains anything else than whitespace or comments.

Such blank mood definitions result in an empty abstract syntax tree
indicated by ->ast of the mood parser's context structure being
NULL. We happily dereference that pointer in mp_eval_row() and die
by the resulting SIGSEGV.

The fix is trivial: simply treat blank moods in the same way as the
dummy mood, i.e. regard each audio file as admissible.