buffer: remove minor perf regression in from()

A performance regression was introduced to Buffer.from() between 6.5.0
and 6.6.0. This commit gains back that regression, but does not solve
the actual issue.

From the IRHydra output it seems v8 is doing something every different
in how the functions are evaluated. When passing an array to
Buffer.from() IRHydra shows 31 code blocks in 6.5.0 and 181 code blocks
in 6.6.0. Investigation still needs to be done to determine the root of
this issue.

lib/buffer.js

@@ -242,8 +242,9 @@ function fromObject(obj) {
   }
 
   if (obj) {
-    if (isArrayBuffer(obj.buffer) || 'length' in obj) {
-      if (typeof obj.length !== 'number' || obj.length !== obj.length) {
+    const obj_length = obj.length;
+    if (isArrayBuffer(obj.buffer) || obj_length) {
+      if (typeof obj_length !== 'number' || obj_length !== obj_length) {
         return new FastBuffer();
       }
       return fromArrayLike(obj);