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.

dw/fltkui.cc

@@ -866,9 +866,9 @@ FltkEntryResource::FltkEntryResource (FltkPlatform *platform, int size,
 {
    this->size = size;
    this->password = password;
-   this->label = label ? dStrdup(label) : NULL;
+   this->label = dStrdup(label);
    this->label_w = 0;
-   this->placeholder = placeholder ? dStrdup(placeholder) : NULL;
+   this->placeholder = dStrdup(placeholder);
 
    initText = NULL;
    editable = false;
@@ -1052,7 +1052,7 @@ FltkMultiLineTextResource::FltkMultiLineTextResource (FltkPlatform *platform,
       MSG_WARN("numRows = %d is set to 1.\n", numRows);
       numRows = 1;
    }
-   this->placeholder = placeholder ? dStrdup(placeholder) : NULL;
+   this->placeholder = dStrdup(placeholder);
 
    init (platform);
 }

dw/hyphenator.cc

@@ -371,7 +371,7 @@ TrieBuilder::~TrieBuilder ()
 void TrieBuilder::insert (const char *key, const char *value)
 {
    dataList->increase ();
-   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key);
+   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key).ptr;
    dataList->getLastRef ()->value = dataZone->strdup (value);
 }