gh-151337: Avoid possible memory leak in _tkinter.c on Windows. (GH-151340)

(cherry picked from commit 71805db)

Co-authored-by: Ivy Xu <fakeshadow1337@gmail.com>

Author: ByteFlowing1337

Misc/NEWS.d/next/Library/2026-06-11-21-43-24.gh-issue-151337.JSVV18.rst

@@ -0,0 +1 @@
+Avoid possible memory leak in ``tkinter.c`` on Windows.

Modules/_tkinter.c

@@ -151,18 +151,20 @@ _get_tcl_lib_path(void)
         }
 
         /* Check expected location for an installed Python first */
-        tcl_library_path = PyUnicode_FromString("\\tcl\\tcl" TCL_VERSION);
-        if (tcl_library_path == NULL) {
+        PyObject* tmp_tcl_library_path = PyUnicode_FromString("\\tcl\\tcl" TCL_VERSION);
+        if (tmp_tcl_library_path == NULL) {
             Py_DECREF(prefix);
             return NULL;
         }
-        tcl_library_path = PyUnicode_Concat(prefix, tcl_library_path);
+        tcl_library_path = PyUnicode_Concat(prefix, tmp_tcl_library_path);
+        Py_DECREF(tmp_tcl_library_path);
         Py_DECREF(prefix);
         if (tcl_library_path == NULL) {
             return NULL;
         }
         stat_return_value = _Py_stat(tcl_library_path, &stat_buf);
         if (stat_return_value == -2) {
+            Py_DECREF(tcl_library_path);
             return NULL;
         }
         if (stat_return_value == -1) {
@@ -177,16 +179,17 @@ _get_tcl_lib_path(void)
             }
             stat_return_value = _Py_stat(tcl_library_path, &stat_buf);
             if (stat_return_value == -2) {
+                Py_DECREF(tcl_library_path);
                 return NULL;
             }
             if (stat_return_value == -1) {
                 /* tcltkDir for a repository build doesn't exist either,
                    reset errno and leave Tcl to its own devices */
                 errno = 0;
-                tcl_library_path = NULL;
+                Py_CLEAR(tcl_library_path);
             }
 #else
-            tcl_library_path = NULL;
+            Py_CLEAR(tcl_library_path);
 #endif
         }
         already_checked = 1;
@@ -718,11 +721,13 @@ Tkapp_New(const char *screenName, const char *className,
         if (!ret && GetLastError() == ERROR_ENVVAR_NOT_FOUND) {
             str_path = _get_tcl_lib_path();
             if (str_path == NULL && PyErr_Occurred()) {
+                Py_DECREF(v);
                 return NULL;
             }
             if (str_path != NULL) {
                 utf8_path = PyUnicode_AsUTF8String(str_path);
                 if (utf8_path == NULL) {
+                    Py_DECREF(v);
                     return NULL;
                 }
                 Tcl_SetVar(v->interp,