BUG/MEDIUM: cli: prevent memory leak on write errors

BUG/MEDIUM: cli: prevent memory leak on write errors

Since the introduction of payload support on the CLI in 1.9-dev1 by
commit abbf60710 ("MEDIUM: cli: Add payload support"), a chunk is
temporarily allocated for the CLI to support defragmenting a payload
passed with a command. However it's only released when passing via
the CLI_ST_END state (i.e. on clean shutdown), but not on errors.
Something as trivial as:

  $ while :; do ncat --send-only -U /path/to/cli <<< "show stat"; done

with a few hundreds of servers is enough see the number of allocated
trash chunks go through the roof in "show pools".

This needs to be backported as far as 2.0.