buffer: atob throw error when the input string is invalid

The specification of `atob` has various different conditions that we need
to abide by. The specific changes that were made:

* `atob` now immediately throws when `undefined`, `false`, or a `number` is
  supplied
* `atob` now strips ASCII whitespace before attempting to decode
* `atob` now validates that the code point's length divided by 4 leaves a
  remainder that is not 1

See: https://infra.spec.whatwg.org/#forgiving-base64-decode

Fixes: nodejs#42646

lib/buffer.js

@@ -1259,7 +1259,28 @@ function atob(input) {
   if (arguments.length === 0) {
     throw new ERR_MISSING_ARGS('input');
   }
-  input = `${input}`;
+
+  if (input === undefined || input === false || typeof input === 'number') {
+    throw lazyDOMException(
+      'The string to be decoded is not correctly encoded.',
+      'ValidationError');
+  }
+
+  // Remove all ASCII whitespace from data.
+  //
+  // See #1 - https://infra.spec.whatwg.org/#forgiving-base64
+  input = `${input}`.replace(/\s/g, '');
+
+  // If data's code point length divides by 4 leaving a remainder of 1, then
+  // return failure.
+  //
+  // See #3 - https://infra.spec.whatwg.org/#forgiving-base64
+  if (input.length % 4 === 1) {
+    throw lazyDOMException(
+      'The string to be decoded is not correctly encoded.',
+      'ValidationError');
+  }
+
   for (let n = 0; n < input.length; n++) {
     if (!ArrayPrototypeIncludes(kForgivingBase64AllowedChars,
                                 StringPrototypeCharCodeAt(input, n)))

test/parallel/test-btoa-atob.js

@@ -15,3 +15,12 @@ throws(() => buffer.btoa(), /TypeError/);
 
 strictEqual(atob(' '), '');
 strictEqual(atob('  YW\tJ\njZA=\r= '), 'abcd');
+
+throws(() => buffer.atob(undefined), /ValidationError/);
+throws(() => buffer.atob(false), /ValidationError/);
+throws(() => buffer.atob(1), /ValidationError/);
+throws(() => buffer.atob(0), /ValidationError/);
+throws(() => buffer.atob('a'), /ValidationError/);
+throws(() => buffer.atob('a '), /ValidationError/);
+throws(() => buffer.atob(' a'), /ValidationError/);
+throws(() => buffer.atob('aaaaa'), /ValidationError/);