BUG/MINOR: http/sample: gmtime/localtime can fail

The man said that gmtime() and localtime() can return a NULL value.
This is not tested. It appears that all the values of a 32 bit integer
are valid, but it is better to check the return of these functions.

However, if the integer move from 32 bits to 64 bits, some 64 values
can be unsupported.
(cherry picked from commit fac9ccfb705702f211f99e67d5f5d5129002086a)
[wt: we only have sample_conv_date() in 1.5]
1 file changed