don't panick on time subtraction failure (#141)

On some machines, the system clock can be faulty and start_time maybe
actually be after the end time. This causes a panic, instead gracefully
just put a None time in `self.last_runtime`

libafl/src/observers/mod.rs

@@ -90,7 +90,7 @@ impl<EM, I, S, Z> HasExecHooks<EM, I, S, Z> for TimeObserver {
         _mgr: &mut EM,
         _input: &I,
     ) -> Result<(), Error> {
-        self.last_runtime = Some(current_time() - self.start_time);
+        self.last_runtime = current_time().checked_sub(self.start_time);
         Ok(())
     }
 }