Fix: free the filter string leaked by type_build_select

[mattmundell]

What

Free the filter string that type_build_select obtains from
filter_term (get->filt_id) (or get->filter_replacement) in its cleanup
block, alongside the other allocations it releases.

Why

type_build_select assigned filter and then freed every other allocation in
the block but never that one, so each get_aggregates with a filt_id leaked
a copy of the filter term.

Testing

Before the change, an authenticated get_aggregates with a filter id

<get_aggregates type="nvt" filt_id="..."/>

made gvmd's stderr report a Direct leak of the filter term, allocated by
filter_term and abandoned by type_build_select:

Direct leak of 75 byte(s) in 3 object(s) allocated from:
    #5 sql_string_internal src/sql.c:824
    #6 sql_string src/sql.c:846
    #7 filter_term_sql src/manage_sql_filters.c:2401
    #8 filter_term src/manage_filter_utils.c:844
    #9 type_build_select src/manage_sql.c:28068
    #10 init_aggregate_iterator src/manage_sql.c:1475
    #11 handle_get_aggregates src/gmp.c:12867
SUMMARY: AddressSanitizer: 75 byte(s) leaked in 3 allocation(s).

The request is sent by a user that cannot resolve the filter, so the filter's
name is not fetched on the response path. After the change the same command
completes with no ASan report.