cache: Check return value from offscreen_cache_get

Currently, SetSurface is called regardless of return value from
offscreen_cache_get. It may happen that bitmap is not found in a
cache from some reason, which may lead to segfaults. Let's check
the returned value before use.

https://github.com/FreeRDP/FreeRDP/issues/4117

diff --git a/libfreerdp/cache/offscreen.c b/libfreerdp/cache/offscreen.c
index df79fc8..93fb08e 100644 (file)
--- a/libfreerdp/cache/offscreen.c
+++ b/libfreerdp/cache/offscreen.c
@@ -101,6 +101,9 @@ static BOOL update_gdi_switch_surface(rdpContext* context,
        {
                rdpBitmap* bmp;
                bmp = offscreen_cache_get(cache->offscreen, switchSurface->bitmapId);
+               if (bmp == NULL)
+                       return FALSE;
+
                bitmap->SetSurface(context, bmp, FALSE);
        }