BUG/MINOR: log: improper NULL return check on utoa_pad()

BUG/MINOR: log: improper NULL return check on utoa_pad()

utoa_pad() is directly fed into tmplog, which is checked for NULL.
First, when NULLs are possible, they should be put into a temp variable
in order to preserve tmplog, and second, this return value can never be
NULL because the value passed is tv_usec/1000 (between "0" and "999")
with a 4-char output. However better fix the check in case this code gets
improperly copy-pasted for another usage later.

Reported-by: Dinko Korunic <dkorunic@reflected.net>