Fix compiler warning (g++ 14) [-Wstringop-overflow]

New compiler warning detected by g++ 14.2.0 building with CMake
  in Release mode:

[...]src/Fl_Help_View.cxx: In member function
  ‘void Fl_Help_View::follow_link(Fl_Help_Link*)’:
[...]src/Fl_Help_View.cxx:2812:18:
  warning: ‘size_t strlcpy(char*, const char*, size_t)’ writing
  4096 bytes into a region of size 2045 overflows the destination
  ... strlcpy(tempptr, linkp->filename, sizeof(temp));
  ...         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

FL/Fl_Help_View.H

@@ -225,7 +225,7 @@ class FL_EXPORT Fl_Help_View : public Fl_Group {        // Help viewer widget
                 atargets_;              ///< Allocated targets
   Fl_Help_Target *targets_;             ///< Targets
 
-  char          directory_[FL_PATH_MAX];///< Directory for current file
+  char          directory_[2 * FL_PATH_MAX + 15]; ///< Directory for current file
   char          filename_[FL_PATH_MAX]; ///< Current filename
   int           topline_,               ///< Top line in document
                 leftline_,              ///< Lefthand position

src/Fl_Help_View.cxx

@@ -2701,7 +2701,7 @@ Fl_Shared_Image *
 Fl_Help_View::get_image(const char *name, int W, int H) {
   const char    *localname;             // Local filename
   char          dir[FL_PATH_MAX];       // Current directory
-  char          temp[2 * FL_PATH_MAX],  // Temporary filename
+  char          temp[3 * FL_PATH_MAX],  // Temporary filename
                 *tempptr;               // Pointer into temporary name
   Fl_Shared_Image *ip;                  // Image pointer...
 
@@ -2798,7 +2798,7 @@ void Fl_Help_View::follow_link(Fl_Help_Link *linkp)
   if (strcmp(linkp->filename, filename_) != 0 && linkp->filename[0])
   {
     char        dir[FL_PATH_MAX];       // Current directory
-    char        temp[2 * FL_PATH_MAX],  // Temporary filename
+    char        temp[3 * FL_PATH_MAX],  // Temporary filename
               *tempptr; // Pointer into temporary filename