Correct a (relatively harmless) use after free in prompt expansion
processing [detected by asan.]
Relatively harmless: as (while incorrect) the way the data is (was)
used more or less guaranteed that the buffer contents would be
unaltered until well after they are (were) no longer wanted (this
is the expanded prompt string, it is just output (or copied into
libedit internal storage) and forgotten.
This should make no visible difference to anyone (not using asan or
similar.)
XXX pullup -8
d1b3ee239b