BUG/MINOR: mworker/cli: check ci_insert() return value in pcli_parse_request()

BUG/MINOR: mworker/cli: check ci_insert() return value in pcli_parse_request()

All ci_insert() calls in pcli_parse_request() were ignoring the return
value, silently miscounting the bytes to forward if an insertion failed.

Add a check on each call and return -1 on failure. In practice this has
no impact: the channel receive machinery enforces a maxrewrite reserve,
so there is always sufficient room in the buffer for these small
prefixes by the time pcli_parse_request() is called.

Must be backported in every maintained versions, the list of available
commands might change.