Always check return value of strftime().

strftime() returns 0 and leaves the contents of the output buffer
unspecified if the output buffer is not big enough.  Thus, one should
check strftime()'s return value.  Until now, OVS has had a few invocations
of strftime() that did not check the return value.  This commit fixes
those.  I believe that the buffers were always large enough in each case,
but it's better to be safe.

Reported-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>