Fix static analyzer errors and warnings

Fix several bugs found by scan-build (more to come).
https://clang-analyzer.llvm.org/scan-build.html

See also current travis-ci build log with static analyzer, e.g.
https://travis-ci.com/fltk/fltk/jobs/285426415
(link may become invalid in the future).

fluid/Fl_Function_Type.cxx

@@ -394,7 +394,7 @@ void Fl_Function_Type::write_code1() {
     }
   }
   
-  if (havewidgets && !child->name()) write_c("  %s* w;\n", subclassname(child));
+  if (havewidgets && child && !child->name()) write_c("  %s* w;\n", subclassname(child));
   indentation += 2;
 }
 

src/Fl_Preferences.cxx

@@ -1476,7 +1476,7 @@ void Fl_Preferences::Node::createIndex() {
   int n = nChildren();
   if (n>NIndex_) {
     NIndex_ = n + 16;
-    index_ = (Node**)realloc(index_, NIndex_*sizeof(Node**));
+    index_ = (Node**)realloc(index_, NIndex_*sizeof(Node*));
   }
   Node *nd;
   int i = 0;

src/Fl_Screen_Driver.cxx

@@ -308,15 +308,21 @@ void Fl_Screen_Driver::rescale_all_windows_from_screen(int screen, float f)
   int i = 0, count = 0; // count top-level windows, except transient scale-displaying window
   Fl_Window *win = Fl::first_window();
   while (win) {
-    if (!win->parent() && (Fl_Window_Driver::driver(win)->screen_num() == screen || rescalable() == SYSTEMWIDE_APP_SCALING) &&
-        win->user_data() != &Fl_Screen_Driver::transient_scale_display) count++;
+    if (!win->parent() &&
+	(Fl_Window_Driver::driver(win)->screen_num() == screen || rescalable() == SYSTEMWIDE_APP_SCALING) &&
+        win->user_data() != &Fl_Screen_Driver::transient_scale_display) {
+      count++;
+    }
     win = Fl::next_window(win);
   }
+  if (count == 0)
+    return;
   Fl_Window **win_array = new Fl_Window*[count];
   win = Fl::first_window(); // memorize all top-level windows
   while (win) {
-    if (!win->parent() && win->user_data() != &Fl_Screen_Driver::transient_scale_display &&
-        (Fl_Window_Driver::driver(win)->screen_num() == screen  || rescalable() == SYSTEMWIDE_APP_SCALING) ) {
+    if (!win->parent() &&
+	(Fl_Window_Driver::driver(win)->screen_num() == screen || rescalable() == SYSTEMWIDE_APP_SCALING) &&
+	win->user_data() != &Fl_Screen_Driver::transient_scale_display) {
       win_array[i++] = win;
     }
     win = Fl::next_window(win);

src/Fl_Tabs.cxx

@@ -204,17 +204,18 @@ int Fl_Tabs::handle(int event) {
     return 1;
   case FL_MOVE: {
     int ret = Fl_Group::handle(event);
-    Fl_Widget *tooltip_widget = Fl_Tooltip::current(), *n = tooltip_widget;
+    Fl_Widget *tooltip_widget = Fl_Tooltip::current();
+    Fl_Widget *n; // initialized later
     int H = tab_height();
-    if ( (H>=0) && (Fl::event_y()>y()+H) )
+    if ( (H >= 0) && (Fl::event_y() > y()+H) )
       return ret;
-    else if ( (H<0) && (Fl::event_y() < y()+h()+H) )
+    else if ( (H < 0) && (Fl::event_y() < y()+h()+H) )
       return ret;
     else {
       n = which(Fl::event_x(), Fl::event_y());
       if (!n) n = this;
     }
-    if (n!=tooltip_widget)
+    if (n != tooltip_widget)
       Fl_Tooltip::enter(n);
     return ret; }
   case FL_FOCUS: