video: Don't rely on memcpy undefined behavior

The C specification states that passing a size of 0 to functions like memcpy is valid, but even if the size is 0 and the function is essentially a no-op, the result when passing any invalid pointers is considered undefined behavior. Don't rely on undefined behavior when copying the display or mode lists.

src/video/SDL_video.c

@@ -634,17 +634,19 @@ SDL_DisplayID SDL_AddVideoDisplay(const SDL_VideoDisplay *display, SDL_bool send
     if (displays) {
         int i;
 
-        /* The display list may contain self-referential pointers to the desktop mode. */
-        SDL_memcpy(displays, _this->displays, _this->num_displays * sizeof(*displays));
-        for (i = 0; i < _this->num_displays; ++i) {
-            if (displays[i].current_mode == &_this->displays[i].desktop_mode) {
-                displays[i].current_mode = &displays[i].desktop_mode;
+        if (_this->displays) {
+            /* The display list may contain self-referential pointers to the desktop mode. */
+            SDL_memcpy(displays, _this->displays, _this->num_displays * sizeof(*displays));
+            for (i = 0; i < _this->num_displays; ++i) {
+                if (displays[i].current_mode == &_this->displays[i].desktop_mode) {
+                    displays[i].current_mode = &displays[i].desktop_mode;
+                }
             }
+
+            SDL_free(_this->displays);
         }
 
-        SDL_free(_this->displays);
         _this->displays = displays;
-
         id = _this->next_object_id++;
         new_display = &displays[_this->num_displays++];
 
@@ -954,15 +956,18 @@ SDL_bool SDL_AddFullscreenDisplayMode(SDL_VideoDisplay *display, const SDL_Displ
             return SDL_FALSE;
         }
 
-        /* Copy the list and update the current mode pointer, if necessary. */
-        SDL_memcpy(modes, display->fullscreen_modes, nmodes * sizeof(*modes));
-        for (i = 0; i < nmodes; ++i) {
-            if (display->current_mode == &display->fullscreen_modes[i]) {
-                display->current_mode = &modes[i];
+        if (display->fullscreen_modes) {
+            /* Copy the list and update the current mode pointer, if necessary. */
+            SDL_memcpy(modes, display->fullscreen_modes, nmodes * sizeof(*modes));
+            for (i = 0; i < nmodes; ++i) {
+                if (display->current_mode == &display->fullscreen_modes[i]) {
+                    display->current_mode = &modes[i];
+                }
             }
+
+            SDL_free(display->fullscreen_modes);
         }
 
-        SDL_free(display->fullscreen_modes);
         display->fullscreen_modes = modes;
         display->max_fullscreen_modes += 32;
     }