lib: fix gmtime_assafe potential issues

lib: fix gmtime_assafe potential issues

Changes:
- Convert `unsigned int` to `time_t` to satisfy time truncation warnings
  even though at this point we had already used the modulus operator.

- Avoid trying to access outside the bounds of the array

  `months` array has a size of 13 elements, but the code inside the loop
  uses `i + 1` to peek on the next month.

Found by Coverity Scan (CID 1519752 and 1519769)

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>