Avoid a potential innocuous compiler warning.

Add a cast to avoid comparing a ssize_t value to a uint64_t value that
is always larger than a 32-bit ssize_t.  This silences an innocuous
compiler warning from e.g. gcc 4.2.1 about the comparison always having
the same result.

src/arena.c

@@ -1352,7 +1352,11 @@ static bool
 arena_decay_time_valid(ssize_t decay_time)
 {
 
-	return (decay_time >= -1 && decay_time <= NSTIME_SEC_MAX);
+	if (decay_time < -1)
+		return (false);
+	if (decay_time == -1 || (uint64_t)decay_time <= NSTIME_SEC_MAX)
+		return (true);
+	return (false);
 }
 
 ssize_t