user32: WM_GETTEXT message converters have to 0-terminate output buffer if there…

dlls/user32/winproc.c

@@ -856,9 +856,11 @@ LRESULT WINPROC_CallProcAtoW( winproc_callback_t callback, HWND hwnd, UINT msg,
 
             if (!(ptr = get_buffer( buffer, sizeof(buffer), len ))) break;
             ret = callback( hwnd, msg, wParam, (LPARAM)ptr, result, arg );
-            if (*result && wParam)
+            if (wParam)
             {
-                RtlUnicodeToMultiByteN( str, wParam - 1, &len, ptr, strlenW(ptr) * sizeof(WCHAR) );
+                len = 0;
+                if (*result)
+                    RtlUnicodeToMultiByteN( str, wParam - 1, &len, ptr, strlenW(ptr) * sizeof(WCHAR) );
                 str[len] = 0;
                 *result = len;
             }
@@ -1091,10 +1093,13 @@ static LRESULT WINPROC_CallProcWtoA( winproc_callback_t callback, HWND hwnd, UIN
 
             if (!(ptr = get_buffer( buffer, sizeof(buffer), len ))) break;
             ret = callback( hwnd, msg, wParam, (LPARAM)ptr, result, arg );
-            if (*result && len)
+            if (len)
             {
-                RtlMultiByteToUnicodeN( (LPWSTR)lParam, wParam*sizeof(WCHAR), &len, ptr, strlen(ptr)+1 );
-                *result = len/sizeof(WCHAR) - 1;  /* do not count terminating null */
+                if (*result)
+                {
+                    RtlMultiByteToUnicodeN( (LPWSTR)lParam, wParam*sizeof(WCHAR), &len, ptr, strlen(ptr)+1 );
+                    *result = len/sizeof(WCHAR) - 1;  /* do not count terminating null */
+                }
                 ((LPWSTR)lParam)[*result] = 0;
             }
             free_buffer( buffer, ptr );