test: increase coverage for punycode's decode

Added test cases for error.

PR-URL: nodejs#10940
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-punycode.js

@@ -18,6 +18,15 @@ assert.strictEqual(punycode.decode(
     'Willst du die Blthe des frhen, die Frchte des spteren Jahres-x9e96lkal'),
     'Willst du die Blüthe des frühen, die Früchte des späteren Jahres');
 assert.strictEqual(punycode.decode('wgv71a119e'), '日本語');
+assert.throws(() => {
+  punycode.decode(' ');
+}, /^RangeError: Invalid input$/);
+assert.throws(() => {
+  punycode.decode('α-');
+}, /^RangeError: Illegal input >= 0x80 \(not a basic code point\)$/);
+assert.throws(() => {
+  punycode.decode('あ');
+}, /^RangeError: Overflow: input needs wider integers to process$/);
 
 // http://tools.ietf.org/html/rfc3492#section-7.1
 const tests = [