free of uninitialized DString in tkWindow.c

in generic/tkWindow.c, near line 2938, there is the
following code:

if (Tcl_IsSafe(interp)) {
<snip>
Tcl_DString ds;
<snip>
while (1) {
master = Tcl_GetMaster(master);
if (master == NULL) {
Tcl_DStringFree(&ds);

I believe the call to Tcl_DStringFree is an error. The
variable "ds" does not actually get initialized until later,
near line 2978. Attempting to free this invalid DString
causes a crash in the debug build of the HEAD. This
same bug also exists in the core-8-4-branch.