parsedate: avoid integer overflow

In C, signed integer overflow is undefined behavior. Thus, the compiler
is allowed to assume that it will not occur. In the check for an
overflow, the developer assumes that the signed integer of type time_t
will wrap around if it overflows. However, this behavior is undefined in
the C standard. Thus, when the compiler sees this, it simplifies t +
delta < t to delta < 0. Since delta > 0 and delta < 0 can't both be
true, the entire if statement is optimized out under certain
optimization levels. Thus, the parsedate function would return
PARSEDATE_OK with an undefined value in the time, instead of return -1 =
PARSEDATE_FAIL.

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