ffplay: drop redundant NULL sws_freeContext() check in stream_close()

sws_freeContext() already checks for NULL, simplify.

ffplay.c

@@ -994,8 +994,7 @@ static void stream_close(VideoState *is)
     SDL_DestroyCond(is->subpq_cond);
     SDL_DestroyCond(is->continue_read_thread);
 #if !CONFIG_AVFILTER
-    if (is->img_convert_ctx)
-        sws_freeContext(is->img_convert_ctx);
+    sws_freeContext(is->img_convert_ctx);
 #endif
     av_free(is);
 }