logger: fix --size use for stdin

logger: fix --size use for stdin

The stdin version counts log header into the message size, but
for example when it reads message from argv[] it counts only message
itself.

 $ logger --stderr  --size 3 "abcd"
 <13>Oct 21 18:48:29 kzak: abc

 $ echo "abcd" | logger --stderr  --size 3
 logger: cannot allocate 18446744073709551597 bytes: Cannot allocate memory

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=2011602
Signed-off-by: Karel Zak <kzak@redhat.com>