Fix Sparse warnings: "Using plain integer as NULL pointer"

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/sdl.c b/sdl.c
index 50a4a01..238ef08 100644
--- a/sdl.c
+++ b/sdl.c
@@ -53,7 +53,7 @@
 static int absolute_enabled = 0;
 static int guest_cursor = 0;
 static int guest_x, guest_y;
-static SDL_Cursor *guest_sprite = 0;
+static SDL_Cursor *guest_sprite = NULL;
 static uint8_t allocator;
 static SDL_PixelFormat host_format;
 static int scaling_active = 0;