test: test isFullWidthCodePoint with invalid input

Code coverage information shows that we are only testing the happy path
for the internal readline `isFullWidthCodePoint()` function. Test it
with invalid input.

PR-URL: https://github.com/nodejs/node/pull/7422
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-readline-interface.js

@@ -304,6 +304,11 @@ function isWarned(emitter) {
     rli.close();
   }
 
+  // isFullWidthCodePoint() should return false for non-numeric values
+  [true, false, null, undefined, {}, [], 'あ'].forEach((v) => {
+    assert.strictEqual(readline.isFullWidthCodePoint('あ'), false);
+  });
+
   // wide characters should be treated as two columns.
   assert.equal(readline.isFullWidthCodePoint('a'.charCodeAt(0)), false);
   assert.equal(readline.isFullWidthCodePoint('あ'.charCodeAt(0)), true);