[SR-7112] Fixed a case where CFStringGetCStringPtr returns a string without null termination

Changed _fastCStringContents (used in CFStringGetCStringPtr) to return NULL if required to return a string containing null termination.
Since this fix, if you call CFStringGetCStringPtr with a Swift-based CFString, it returns NULL.

Before the change: _fastCStringContents always returned a pointer to an ASCII string containing no NULL termination.
This is because the code was using unsafeBitCast to get internal pointer of String of Swift. Swift's string does not guarantee null termination.

Author: tid-kijyun

Foundation/NSString.swift

@@ -294,6 +294,10 @@ open class NSString : NSObject, NSCopying, NSMutableCopying, NSSecureCoding, NSC
     }
 
     internal func _fastCStringContents(_ nullTerminated: Bool) -> UnsafePointer<Int8>? {
+        guard !nullTerminated else {
+            // There is no way to fastly and safely retrieve a pointer to a null-terminated string from a String of Swift.
+            return nil
+        }
         if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
             if _storage._guts._isContiguousASCII {
                 return unsafeBitCast(_storage._guts.startASCII, to: UnsafePointer<Int8>.self)