buffer: fix single digit hex string handling

Fixes single digit hex strings not raising TypeError on Buffer creation.

Fixes: https://github.com/nodejs/node/issues/6770
PR-URL: https://github.com/nodejs/node/pull/6775
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/buffer.js

@@ -225,11 +225,11 @@ function fromString(string, encoding) {
   if (!Buffer.isEncoding(encoding))
     throw new TypeError('"encoding" must be a valid string encoding');
 
-  var length = byteLength(string, encoding);
-
-  if (length === 0)
+  if (string.length === 0)
     return Buffer.alloc(0);
 
+  var length = byteLength(string, encoding);
+
   if (length >= (Buffer.poolSize >>> 1))
     return binding.createFromString(string, encoding);
 

test/parallel/test-buffer.js

@@ -749,6 +749,15 @@ for (let i = 0; i < 256; i++) {
   assert.equal(hexb2[i], hexb[i]);
 }
 
+// Test single hex character throws TypeError
+// - https://github.com/nodejs/node/issues/6770
+assert.throws(function() {
+  Buffer.from('A', 'hex');
+}, TypeError);
+
+// Test single base64 char encodes as 0
+assert.strictEqual(Buffer.from('A', 'base64').length, 0);
+
 {
   // test an invalid slice end.
   console.log('Try to slice off the end of the buffer');