EGL: Fixed SDL_GL_GetSwapInterval() returning -1 on error.

It should return 0 as a safe default on errors. -1 is returned for late swaps.

src/video/SDL_egl.c

@@ -558,7 +558,8 @@ int
 SDL_EGL_GetSwapInterval(_THIS)
 {
     if (!_this->egl_data) {
-        return SDL_SetError("EGL not initialized");
+        SDL_SetError("EGL not initialized");
+        return 0;
     }
 
     return _this->egl_data->egl_swapinterval;