Odd valgrind error in imm.c, possible fix

[Dan Kegel]

Today a test got stuck, and I had to kill wineserver.
After that, lots of tests seem to have the same
error:

+ Invalid read of size 4
+    at  IMM_FreeThreadData (imm.c:233)
+    by  DllMain (imm.c:385)
+    by  __wine_spec_dll_entry (dll_entry.c:40)
+    by  (within /home/dank/wine-git/dlls/ntdll/ntdll.dll.so)
+    by  MODULE_InitDLL (loader.c:910)
+    by  process_detach (loader.c:1081)
+    by  LdrShutdownProcess (loader.c:2143)
+    by  ExitProcess (process.c:2101)
+    by  service_run_main_thread (service.c:604)
+    by  StartServiceCtrlDispatcherW (service.c:703)
+    by  wmain (device.c:287)
+  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Presumably this is because killing wineserver also killed
the background notepad I run to keep from having
to valgrind wineboot etc. over and over again.
If the immediate cause is something like
imm32's DllMain getting called with DLL_PROCESS_DETACH
after being called with DLL_THREAD_DETACH, the following
patch might be a sensible fix.  Is it?

diff --git a/dlls/imm32/imm.c b/dlls/imm32/imm.c
index 94b7752..bcd11f1 100644
--- a/dlls/imm32/imm.c
+++ b/dlls/imm32/imm.c
@@ -230,10 +230,13 @@ static void IMM_InitThreadData(void)
 static void IMM_FreeThreadData(void)
 {
     IMMThreadData* data = TlsGetValue(tlsIndex);
-    IMM_DestroyContext(data->defaultContext);
-    DestroyWindow(data->hwndDefault);
-    HeapFree(GetProcessHeap(),0,data);
-    TRACE("Thread Data Destroyed\n");
+    if (data)
+    {
+        IMM_DestroyContext(data->defaultContext);
+        DestroyWindow(data->hwndDefault);
+        HeapFree(GetProcessHeap(),0,data);
+        TRACE("Thread Data Destroyed\n");
+    }
 }

static HMODULE LoadDefaultWineIME(void)