Foundation: fix missing string terminator

The computation of the base64encoding would trim the trailing nul
terminator on the rendered string.  This would result in invalid
string manipulation leading to heap corruptions on Windows.

Thanks to Simon Evans for spotting the issue and proposing the fix!
(And a thank you for spotting a leak that I was accidentally
introducing.)

Author: compnerd

Foundation/NSData.swift

@@ -597,12 +597,15 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
         let dataLength = self.length
         if dataLength == 0 { return "" }
 
-        let capacity = estimateBase64Size(length: dataLength)
+        let capacity = estimateBase64Size(length: dataLength) + 1
         let ptr = UnsafeMutableRawPointer.allocate(byteCount: capacity, alignment: 1)
         let buffer = UnsafeMutableRawBufferPointer(start: ptr, count: capacity)
         let length = NSData.base64EncodeBytes(self, options: options, buffer: buffer)
 
-        return String(bytesNoCopy: ptr, length: length, encoding: .ascii, freeWhenDone: true)!
+        buffer[length] = 0
+        let string = String(validatingUTF8: ptr.assumingMemoryBound(to: CChar.self))!
+        ptr.deallocate()
+        return string
     }
 
     /// Creates a Base64, UTF-8 encoded Data from the data object using the given options.