Don't assume localtime() result is non-nil (apple#280)

Don't assume localtime() result is non-nil

Author: gwynne

Sources/Logging/Logging.swift

@@ -1388,7 +1388,9 @@ public struct StreamLogHandler: LogHandler {
         _ = strftime(&buffer, buffer.count, "%Y-%m-%dT%H:%M:%S%z", &localTime)
         #else
         var timestamp = time(nil)
-        let localTime = localtime(&timestamp)
+        guard let localTime = localtime(&timestamp) else {
+            return "<unknown>"
+        }
         strftime(&buffer, buffer.count, "%Y-%m-%dT%H:%M:%S%z", localTime)
         #endif
         return buffer.withUnsafeBufferPointer {