unpack_trees(): minor memory leak fix in unused destination index

This adds a "discard_index(&o->result)" to the failure path, to reclaim memory from an in-core index we built but ended up not using. The *big* memory leak comes from the fact that we leak the cache_entry things left and right. That's a very traditional and deliberate leak: because we used to build up the cache entries by just mapping them directly in from the index file (and we emulate that in modern times by allocating them from one big array), we can't actually free them one-by-one. So doing the "discard_index()" will free the hash tables etc, which is good, and it will free the "istate->alloc" but that is never set on the result because we don't get the result from the index read. So we don't actually free the individual cache entries themselves that got created from the trees. That's not something new, btw. We never did. But some day we should just add a flag to the cache_entry() that it's a "free one by one" kind, and then we could/should do it. In the meantime, this one-liner will fix *some* of the memory leaks, but not that old traditional one. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>