There could be an off by one error in the decoding of PYTHONEXECUTABLE, mbstowcs will only add a NUL character when there is room for one and when the environment variable is all ASCII the buffer isn't large enough which means the argument to Py_SetProgramName is not NUL-terminated unless there happens to be a NUL byte just beyond the allocated buffer.

The attached patch ensures that there is room for a NUL character. I don't know if this actually fixes the crash as I've been unable to reproduce the issue (with various values of PYTHONEXECUBLE and with/without malloc checking environment variables (see malloc(3)).