filter: Handle bad filter numbers gracefully.

filter: Handle bad filter numbers gracefully.

This modifies filter_get() to return NULL if an invalid filter number
is given, rather than aborting the process. This way applications
can loop over all filters without having to include filter_cmd.lsg.h.

filter_name(), which also receives a filter number, has no such
sanity check and could possibly access uninitialized memory if an
invalid argument was passed. This commit adds the check and makes the
function return NULL in this case, just like filter_get(). Moreover,
the function lacked documentation, so let's add it now.