BUG/MINOR: log: fixing bug in tcp syslog_io_handler Octet-Counting

BUG/MINOR: log: fixing bug in tcp syslog_io_handler Octet-Counting

syslog_io_handler does specific treatment to handle syslog tcp octet
counting:

Logic was good, but a sneaky mistake prevented
rfc-6587 octet counting from working properly.

trash.area was used as an input buffer.
It does not make sense here since it is uninitialized.
Compilation was unaffected because trash is a thread
local "global" variable.

buf->area should definitely be used instead.

This should be backported as far as 2.4.