BUILD/MINOR: tools: fix build warning in the date conversion functions

BUILD/MINOR: tools: fix build warning in the date conversion functions

Some gcc versions emit potential null deref warnings at -O3 in
date2str_log(), gmt2str_log() and localdate2str_log() after utoa_pad()
because this function may return NULL if its size argument is too small
for the integer value. And it's true that we can't guarantee that the
input number is always valid.

This must be backported to all stable versions.