notmuch search: Clean up some memory leaks during search loop.

With talloc, we were already freeing all memory by the time we exited
the loop, but that didn't help with excess use of memory inside the
loop, (which was mostly from tallocing some objects with the incorrect
parent).

Thanks to Andrew Tridgell for sitting next to me and teaching me to
use talloc_report_full to find these leaks.