fix valgrind warning in test_display__stream

create_test_primary_surface::test_display_base.c creates a
QXLDevSurfaceCreate structure and initialize it, but doesn't set
the position field. Moreover, this structure has 4 bytes of padding
to the end (as shown by pahole from dwarves), so initialize the whole
structure to 0 before using it.

server/tests/test_display_base.c

@@ -235,7 +235,7 @@ static uint8_t primary_surface[HEIGHT * WIDTH * 4];
 
 static void create_test_primary_surface(QXLWorker *worker)
 {
-    QXLDevSurfaceCreate surface;
+    QXLDevSurfaceCreate surface = { 0, };
 
     surface.format     = SPICE_SURFACE_FMT_32_xRGB;
     surface.width      = WIDTH;