do not die in freeimage(0) when screen==0

src/libdraw/alloc.c

diff --git a/src/libdraw/alloc.c b/src/libdraw/alloc.c
index 53e9605..fb43351 100644
--- a/src/libdraw/alloc.c
+++ b/src/libdraw/alloc.c
@@ -231,9 +231,10 @@
 {
 	int ret;
 
+	if(i == nil)
+		return;
 	if(i == screen)
 		abort();
-
 	ret = _freeimage1(i);
 	free(i);
 	return ret;