[PATCH] user32: Protect DefMDIChildProcA/W from invalid window handles.

[Dmitry Timoshkov]

This patch fixes intermittent crashes in a multithreaded MDI application
I'm working on.
---
 dlls/user32/mdi.c |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/dlls/user32/mdi.c b/dlls/user32/mdi.c
index 7e017b0..7904123 100644
--- a/dlls/user32/mdi.c
+++ b/dlls/user32/mdi.c
@@ -1441,12 +1441,18 @@ LRESULT WINAPI DefFrameProcW( HWND hwnd, HWND hwndMDIClient,
 LRESULT WINAPI DefMDIChildProcA( HWND hwnd, UINT message,
                                    WPARAM wParam, LPARAM lParam )
 {
-    HWND client = GetParent(hwnd);
-    MDICLIENTINFO *ci = get_client_info( client );
+    HWND client;
+    MDICLIENTINFO *ci;
+
+    if (!IsWindow( hwnd )) return 0;
 
     TRACE("%p %04x (%s) %08lx %08lx\n", hwnd, message, SPY_GetMsgName(message, hwnd), wParam, lParam);
 
     hwnd = WIN_GetFullHandle( hwnd );
+
+    client = GetParent( hwnd );
+    ci = get_client_info( client );
+
     if (!ci) return DefWindowProcA( hwnd, message, wParam, lParam );
 
     switch (message)
@@ -1481,12 +1487,18 @@ LRESULT WINAPI DefMDIChildProcA( HWND hwnd, UINT message,
 LRESULT WINAPI DefMDIChildProcW( HWND hwnd, UINT message,
                                    WPARAM wParam, LPARAM lParam )
 {
-    HWND client = GetParent(hwnd);
-    MDICLIENTINFO *ci = get_client_info( client );
+    HWND client;
+    MDICLIENTINFO *ci;
+
+    if (!IsWindow( hwnd )) return 0;
 
     TRACE("%p %04x (%s) %08lx %08lx\n", hwnd, message, SPY_GetMsgName(message, hwnd), wParam, lParam);
 
     hwnd = WIN_GetFullHandle( hwnd );
+
+    client = GetParent( hwnd );
+    ci = get_client_info( client );
+
     if (!ci) return DefWindowProcW( hwnd, message, wParam, lParam );
 
     switch (message)
-- 
1.6.2.4