Fix SolarMutex not locked when using IME on dropdown list.

Surround ImplHandleIMEQueryCharPosition() by
ImplSalYieldMutexAcquireWithWait() and ImplSalYieldMutexRelease().

Change-Id: I3843ad351f3b92801cd1e0066a3c73f2a52c44bd
Reviewed-on: https://gerrit.libreoffice.org/27117Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mark Hung <marklh9@gmail.com>

vcl/win/window/salframe.cxx

@@ -5857,7 +5857,13 @@ LRESULT CALLBACK SalFrameWndProc( HWND hWnd, UINT nMsg, WPARAM wParam, LPARAM lP
             }
             else if ( (sal_uIntPtr)( wParam ) == IMR_QUERYCHARPOSITION )
             {
-                nRet = ImplHandleIMEQueryCharPosition( hWnd, lParam );
+                if ( ImplSalYieldMutexTryToAcquire() )
+                {
+                    nRet = ImplHandleIMEQueryCharPosition( hWnd, lParam );
+                    ImplSalYieldMutexRelease();
+                }
+                else
+                    nRet = FALSE;
                 rDef = FALSE;
             }
             break;