table plugin: Assert that result_num is zero when result is NULL.

table plugin: Assert that result_num is zero when result is NULL.

The Coverity check gets confused by the `tbl->results == NULL` check:
due to the check, is assumes that tbl->results *may* be NULL. It then
discovers the dereference in tbl_clear() without realizing that "result"
and "result_num" are linked and that situation can't happen. This
assertion makes this link a bit easier to conclude.

CID: 37966