:set prompt λ makes WinGHCI unable to start.

```
What steps will reproduce the problem?
1. Enter ":set prompt λ"
2. You will see an error*, close WinGHCI and start it again.
3. Its state will be "not responding".

What is the expected output? What do you see instead?
I expect it to work, but it hangs.

What version of the product are you using? On what operating system?
WinGHCI 1.0.6 on Windows 8.1 x64

Please provide any additional information below.
Probably because of bad Unicode support. The fix is to open regedit, navigate
to HKEY_CURRENT_USER\SOFTWARE\Haskell\WinGHCi and delete
GHCi_PROMPT_COMBO(number here) REG_SZ λ . For more info see
http://stackoverflow.com/questions/14280764/winghci-wont-start

*The error is:
Prelude> :set prompt λ
: hPutChar: invalid argument (invalid character)

```

Original issue reported on code.google.com by `grzesiek...@op.pl` on 26 Jan 2014 at 8:19

Reproduce the failure in WinGHCI 1.0.6 on Windows with `:set prompt λ`, then inspect how the prompt is stored under `HKEY_CURRENT_USER\SOFTWARE\Haskell\WinGHCi`. Confirm that restarting after the invalid character does not hang, while preserving valid prompt settings and handling the reported `hPutChar: invalid argument` error.

Written by the indexing model from the issue text.