Simplify gdk_monitor_get_model string duplication

* src/xfns.c (x-display-monitor-attributes-list): Use dupstring over
explicit if/xstrdup.

* src/frame.c (free_monitors): Remove redundant check for NULL.

src/frame.c

@@ -5670,8 +5670,7 @@ free_monitors (struct MonitorInfo *monitors, int n_monitors)
 {
   int i;
   for (i = 0; i < n_monitors; ++i)
-    if (monitors[i].name)
-      xfree (monitors[i].name);
+    xfree (monitors[i].name);
   xfree (monitors);
 }
 # endif

src/xfns.c

@@ -5030,8 +5030,7 @@ Internal use only, use `display-monitor-attributes-list' instead.  */)
       mi->mm_height = height_mm;
 
 #if GTK_CHECK_VERSION (3, 22, 0)
-      if (gdk_monitor_get_model (monitor))
-        mi->name = xstrdup (gdk_monitor_get_model (monitor));
+      dupstring (&mi->name, (gdk_monitor_get_model (monitor)));
 #elif GTK_CHECK_VERSION (2, 14, 0)
       mi->name = gdk_screen_get_monitor_plug_name (gscreen, i);
 #endif