[crater] Don't serialize nans

This would happen in certain cases where some error caused every target
to fail. This hasn't happened in production but it has happened various
times in my local testing, and it's annoying.

fontc_crater/src/ttx_diff_runner.rs

@@ -107,8 +107,9 @@ impl Summary {
             })
             .sum::<f32>();
         let total_diff = total_diff + (identical as f32);
-        let diff_perc_including_failures = total_diff / (n_failed + success.len()) as f32 * 100.;
-        let diff_perc_excluding_failures = total_diff / success.len() as f32 * 100.;
+        let diff_perc_including_failures =
+            non_nan(total_diff / (n_failed + success.len()) as f32) * 100.;
+        let diff_perc_excluding_failures = non_nan(total_diff / success.len() as f32) * 100.;
         let (mut fontc_failed, mut fontmake_failed, mut both_failed, mut other_failure) =
             (0, 0, 0, 0);
         for fail in failure.values() {
@@ -146,6 +147,15 @@ fn assert_has_timeout_coreutil() {
     std::process::exit(1);
 }
 
+// replace nan with 0
+fn non_nan(val: f32) -> f32 {
+    if val.is_nan() {
+        0.0
+    } else {
+        val
+    }
+}
+
 /// make sure we can find and execute ttx_diff script
 pub(super) fn assert_can_run_script() {
     // first check that we can find timeout(1) (not present on macOS by default,