buffer: remove support for unknown objects

So far objects with a `length` property that was not an integer or
positive resulted in an empty buffer being created. That is not ideal
as it's not really clear what the intention in was.
This throws an error from now on instead.

lib/buffer.js

@@ -28,6 +28,7 @@ const {
   MathFloor,
   MathMin,
   MathTrunc,
+  NumberIsInteger,
   NumberIsNaN,
   NumberMAX_SAFE_INTEGER,
   NumberMIN_SAFE_INTEGER,
@@ -488,10 +489,8 @@ function fromObject(obj) {
     return b;
   }
 
-  if (obj.length !== undefined || isAnyArrayBuffer(obj.buffer)) {
-    if (typeof obj.length !== 'number') {
-      return new FastBuffer();
-    }
+  if ((NumberIsInteger(obj.length) && obj.length >= 0) ||
+      isAnyArrayBuffer(obj.buffer)) {
     return fromArrayLike(obj);
   }
 

test/parallel/test-buffer-alloc.js

@@ -769,8 +769,14 @@ Buffer.allocUnsafe(3.3).fill().toString();
 // Throws bad argument error in commit 43cb4ec
 Buffer.alloc(3.3).fill().toString();
 assert.strictEqual(Buffer.allocUnsafe(3.3).length, 3);
-assert.strictEqual(Buffer.from({ length: 3.3 }).length, 3);
-assert.strictEqual(Buffer.from({ length: 'BAM' }).length, 0);
+assert.throws(
+  () => Buffer.from({ length: 3.3 }),
+  { code: 'ERR_INVALID_ARG_TYPE' }
+);
+assert.throws(
+  () => Buffer.from({ length: 'BAM' }),
+  { code: 'ERR_INVALID_ARG_TYPE' }
+);
 
 // Make sure that strings are not coerced to numbers.
 assert.strictEqual(Buffer.from('99').length, 2);
@@ -993,10 +999,10 @@ assert.strictEqual(SlowBuffer.prototype.offset, undefined);
   // Test that large negative Buffer length inputs don't affect the pool offset.
   // Use the fromArrayLike() variant here because it's more lenient
   // about its input and passes the length directly to allocate().
-  assert.deepStrictEqual(Buffer.from({ length: -Buffer.poolSize }),
-                         Buffer.from(''));
-  assert.deepStrictEqual(Buffer.from({ length: -100 }),
-                         Buffer.from(''));
+  assert.throws(
+    () => Buffer.from({ length: -Buffer.poolSize }),
+    { code: 'ERR_INVALID_ARG_TYPE' }
+  );
 
   // Check pool offset after that by trying to write string into the pool.
   Buffer.from('abc');

test/parallel/test-buffer-arraybuffer.js

@@ -150,4 +150,7 @@ assert.throws(function() {
 }
 
 // Test an array like entry with the length set to NaN.
-assert.deepStrictEqual(Buffer.from({ length: NaN }), Buffer.alloc(0));
+assert.throws(
+  () => Buffer.from({ length: NaN }),
+  { code: 'ERR_INVALID_ARG_TYPE' }
+);