Fix crash in UI with nan/inf/-inf

Veusz version: 1.21
Python version: 2.7.6 (default, Nov 10 2013, 19:24:18) [MSC v.1500 32 bit (Intel)]
Python platform: win32
Numpy version: 1.8.0
Qt version: 4.8.5
PyQt version: 4.10.3
SIP version: 4.15.3
Date: Mon, 07 Jul 2014 11:50:53 +0000

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.dialogs.dataeditdialog", line 454, in setEditorData
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.setting.settingdb", line 221, in ui_floattostring
ValueError: need more than 1 value to unpack