Fix logprint time_t handling

Fix logprint time_t handling

In JSON formatting, I passed to localtime an int variable cast to
time_t. This is UB, as the size of time_t is not defined (in fact,
time_t doesn't even need to be an integer in C). This commit fixes this
by assigning the data to a temporary time_t variable.

Also, such a system was already present when formatting to other
formats, but it featured an unnecessary cast, which this commit removes.

Change-Id: I75d30cf211b9d05c833a798070cf3c0a445956ef