Fix memory corruption with NSMutableData(length:) (#383)

addition of testcase

Author: mbvreddy

Foundation/NSData.swift

@@ -936,9 +936,7 @@ extension NSMutableData {
     }
 
     public convenience init?(length: Int) {
-        let memory = malloc(length)
-        self.init(bytes: memory, length: length, copy: false) { buffer, amount in
-            free(buffer)
-        }
+        self.init(bytes: nil, length: 0)
+        self.length = length
     }
 }

TestFoundation/TestNSData.swift

@@ -39,7 +39,8 @@ class TestNSData: XCTestCase {
             ("test_initializeWithBase64EncodedStringGetsDecodedData", test_initializeWithBase64EncodedStringGetsDecodedData),
             ("test_base64DecodeWithPadding1", test_base64DecodeWithPadding1),
             ("test_base64DecodeWithPadding2", test_base64DecodeWithPadding2),
-            ("test_rangeOfData",test_rangeOfData)
+            ("test_rangeOfData",test_rangeOfData),
+            ("test_initMutableDataWithLength", test_initMutableDataWithLength)
         ]
     }
 
@@ -338,4 +339,10 @@ class TestNSData: XCTestCase {
 
     }
 
+    func test_initMutableDataWithLength() {
+        let mData = NSMutableData(length: 30)
+        XCTAssertNotNil(mData)
+        XCTAssertEqual(mData!.length, 30)
+    }
+
 }