Placate static analysis with an assertion.

src/video/SDL_video.c

@@ -2055,6 +2055,7 @@ SDL_SetWindowGammaRamp(SDL_Window * window, const Uint16 * red,
         if (SDL_GetWindowGammaRamp(window, NULL, NULL, NULL) < 0) {
             return -1;
         }
+        SDL_assert(window->gamma != NULL);
     }
 
     if (red) {