Fix UB from out-of-range <stretch>

If there was a scaling mode value (serialized in the XML as <stretch>
for legacy reasons) that was not 0 or 1 or 2, then the rectangle with
the stretch information would not be initialized by get_stretch_info,
which would lead to a crash, either from dividing by zero (most likely)
or from reading an uninitialized value.

To fix this, when reading <stretch>, normalize it to a sane default if
the value is otherwise bogus. And for good measure, an assertion is
added in get_stretch_info() if the value is still somehow bogus.

Fixes #1155.

desktop_version/src/Graphics.cpp

@@ -3517,6 +3517,13 @@ void Graphics::get_stretch_info(SDL_Rect* rect)
         rect->w = width;
         rect->h = height;
         break;
+    default:
+        SDL_assert(0 && "Invalid scaling mode!");
+        /* Width and height should be nonzero to avoid division by zero. */
+        rect->x = 0;
+        rect->y = 0;
+        rect->w = width;
+        rect->h = height;
     }
 }