readEntries had two memory bugs on the allocation failure path, both
only reachable under OOM:
- The map itself was never freed if we ran into an allocation failure
- The unconditional `errdefer`s for the dupe'd hostname and terminfo
values could double-free if there was a later allocation failure.
This change restructures this function so that these values are dupe'd
up-front, and then their ownership is tracked using optionals that can
be null'ed out once their ownership is transferred into the map.
Both of these cases are now covered by unit tests.
a5550a2dcb