buffer: fix backwards incompatibility

4a86803 introduced a backwards incompatibility by accident
and was not caught due to an existing test that wasn't strict enough.

This commit fixes both the backwards incompatibility and the test.

PR-URL: #12439
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/buffer.js

@@ -456,10 +456,7 @@ function byteLength(string, encoding) {
         return len >>> 1;
       break;
   }
-  if (mustMatch)
-    throw new TypeError('Unknown encoding: ' + encoding);
-  else
-    return binding.byteLengthUtf8(string);
+  return (mustMatch ? -1 : binding.byteLengthUtf8(string));
 }
 
 Buffer.byteLength = byteLength;

test/parallel/test-buffer-alloc.js

@@ -885,7 +885,8 @@ assert.throws(() => Buffer.allocUnsafe(8).writeFloatLE(0.0, -1), RangeError);
 }
 
 // Regression test for #5482: should throw but not assert in C++ land.
-assert.throws(() => Buffer.from('', 'buffer'), TypeError);
+assert.throws(() => Buffer.from('', 'buffer'),
+              /^TypeError: "encoding" must be a valid string encoding$/);
 
 // Regression test for #6111. Constructing a buffer from another buffer
 // should a) work, and b) not corrupt the source buffer.