commit-reach(paint_down_to_common): plug two memory leaks

commit-reach(paint_down_to_common): plug two memory leaks

When a commit is missing, we return early (currently pretending that no
merge basis could be found in that case). At that stage, it is possible
that a merge base could have been found already, and added to the
`result`, which is now leaked.

The priority queue has a similar issue: There might still be a commit in
that queue.

Let's release both, to address the potential memory leaks.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>