assert: fix misformatted error message

Before: `Missing expected exception..`

Afer: `Missing expected exception.`

Author: Trott

lib/assert.js

@@ -342,7 +342,7 @@ function _throws(shouldThrow, block, expected, message) {
   actual = _tryBlock(block);
 
   message = (expected && expected.name ? ' (' + expected.name + ').' : '.') +
-            (message ? ' ' + message : '.');
+            (message ? ' ' + message : '');
 
   if (shouldThrow && !actual) {
     fail(actual, expected, 'Missing expected exception' + message);

test/parallel/test-assert.js

@@ -534,7 +534,7 @@ testAssertionMessage({a: NaN, b: Infinity, c: -Infinity},
     });
   } catch (e) {
     threw = true;
-    assert.strictEqual(e.message, 'Missing expected exception..');
+    assert.strictEqual(e.message, 'Missing expected exception.');
   }
   assert.ok(threw);
 }