String alloc shouldn't convert to std::string.

As I work through making code use more C++ RAII and such, most
of the work is handling strings, especially temporaries.  As member
variables which manage string memory get turned into `std::string`,
some use cases might wind up leaking memory.  (One was found in
this change.)

By using a non-convertible-to-string result, such accidents should
be avoided.

lout/object.cc

@@ -290,7 +290,7 @@ void ConstString::intoStringBuffer(misc::StringBuffer *sb) const
 //    String
 // ------------
 
-String::String (const char *str): ConstString (str ? dStrdup(str) : NULL)
+String::String (const char *str): ConstString (dStrdup(str))
 {
 }