BUG/MINOR: log: fix lf_text_len() truncate inconsistency

BUG/MINOR: log: fix lf_text_len() truncate inconsistency

In c5bff8e550 ("BUG/MINOR: log: improper behavior when escaping log data")
we fixed lf_text_len() behavior with +E (escape) option.

However we introduced an inconsistency if output buffer is too small to
hold the whole output and truncation occurs: indeed without +E option up
to <size> bytes (including NULL byte) will be used whereas with +E option
only <size-1> bytes will be used. Fixing the function and related comment
so that the function behaves the same in regards to truncation whether +E
option is used or not.

This should be backported to all stable versions.