Use after free in getpath_isxfile() on Windows

[serhiy-storchaka]

getpath_isxfile() in Modules/getpath.c frees the path buffer before the last use:

    DWORD attr = GetFileAttributesW(path);
    PyMem_Free(path);
    isxfile = (attr != INVALID_FILE_ATTRIBUTES) &&
              !(attr & FILE_ATTRIBUTE_DIRECTORY) &&
              (cchPath >= 4) &&
              (CompareStringOrdinal(path + cchPath - 4, -1, L".exe", -1, 1 /* ignore case */) == CSTR_EQUAL);

CompareStringOrdinal() reads path + cchPath - 4 after PyMem_Free(path), so the result of the .exe check depends on freed memory. The function is called during interpreter startup, when looking for the executable.

Only the main branch is affected: the early free was introduced in GH-153230, before which the buffer was freed at the end of the block.

[serhiy-storchaka]

The fix was in #158132.