Refine condition for updating minimum path (#17)

Recent change makes the case faster and on a modern CPU it generates unstable results running `cargo test`
* pass
* fail `thread 'year2016::day17::part1_test' panicked at tests\year2016\day17.rs:8:5` with either one of the following failures
  * failure 1: left: "DRRULDDULRDLUURLRLDRRURLRLDDRD" right: "DDRRRD"
  * failrue 2: left: "" right: "DDRRRD"

Add a check to the condition.

Author: boltomli

src/year2016/day17.rs

@@ -69,7 +69,7 @@ fn worker(shared: &Shared) {
         let mut state = shared.mutex.lock().unwrap();
 
         // Update min and max paths.
-        if state.min.is_empty() || local.min.len() < state.min.len() {
+        if !local.min.is_empty() && (state.min.is_empty() || local.min.len() < state.min.len()) {
             state.min.clone_from(&local.min);
         }
         state.max = state.max.max(local.max);