user: Make sure the desktop window exists before calling monitor functions.

dlls/user/misc.c

diff --git a/dlls/user/misc.c b/dlls/user/misc.c
index 4cc1105..842819b 100644
--- a/dlls/user/misc.c
+++ b/dlls/user/misc.c
@@ -396,6 +396,9 @@
 {
     struct monitor_enum_info info;
 
+    /* make sure the desktop window exists */
+    GetDesktopWindow();
+
     info.rect         = *rect;
     info.max_area     = 0;
     info.min_distance = ~0u;