src: fix negative values in process.hrtime()

Fix a regression introduced in commit 89f056b ("node: improve
performance of hrtime()") where the nanosecond field sometimes
had a negative value when calculating the difference between two
timestamps.

Fixes: #4751

src/node.js

@@ -193,10 +193,9 @@
 
       if (typeof ar !== 'undefined') {
         if (Array.isArray(ar)) {
-          return [
-            (hrValues[0] * 0x100000000 + hrValues[1]) - ar[0],
-            hrValues[2] - ar[1]
-          ];
+          const sec = (hrValues[0] * 0x100000000 + hrValues[1]) - ar[0];
+          const nsec = hrValues[2] - ar[1];
+          return [nsec < 0 ? sec - 1 : sec, nsec < 0 ? nsec + 1e9 : nsec];
         }
 
         throw new TypeError('process.hrtime() only accepts an Array tuple');

test/parallel/test-process-hrtime.js

@@ -24,3 +24,6 @@ function validateTuple(tuple) {
     assert(isFinite(v));
   });
 }
+
+const diff = process.hrtime([0, 1e9-1]);
+assert(diff[1] >= 0);  // https://github.com/nodejs/node/issues/4751