src: fix decoding base64 with whitespace

`max_i` should also include the characters that were just read by
`base64_decode_group_slow()`.

PR-URL: #13660
Fixes: #13636
Fixes: #13657
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/base64.h

@@ -99,10 +99,9 @@ size_t base64_decode_fast(char* const dst, const size_t dstlen,
         unbase64(src[i + 3]);
     // If MSB is set, input contains whitespace or is not valid base64.
     if (v & 0x80808080) {
-      const size_t old_i = i;
       if (!base64_decode_group_slow(dst, dstlen, src, srclen, &i, &k))
         return k;
-      max_i = old_i + (srclen - i) / 4 * 4;  // Align max_i again.
+      max_i = i + (srclen - i) / 4 * 4;  // Align max_i again.
     } else {
       dst[k + 0] = ((v >> 22) & 0xFC) | ((v >> 20) & 0x03);
       dst[k + 1] = ((v >> 12) & 0xF0) | ((v >> 10) & 0x0F);

test/parallel/test-buffer-alloc.js

@@ -468,6 +468,10 @@ assert.strictEqual(Buffer.from('=bad'.repeat(1e4), 'base64').length, 0);
 assert.deepStrictEqual(Buffer.from('w0  ', 'base64'),
                        Buffer.from('w0', 'base64'));
 
+// Regression test for https://github.com/nodejs/node/issues/13657.
+assert.deepStrictEqual(Buffer.from(' YWJvcnVtLg', 'base64'),
+                       Buffer.from('YWJvcnVtLg', 'base64'));
+
 {
   // Creating buffers larger than pool size.
   const l = Buffer.poolSize + 5;