Fix crash in CString's 'debugDescription' when the underlying pointer is null.

Fixes <rdar://problem/16227050>

Swift SVN r18202

stdlib/core/CString.swift

@@ -68,6 +68,9 @@ struct CString :
 
 extension CString : DebugPrintable {
   var debugDescription: String {
+    if isNull() {
+      return "<null C string>"
+    }    
     return String.fromCString(self).debugDescription
   }
 }