Fix crash when interatively changing broken axis ranges.

This does not actually make the broken axis have a sensible range, so
should consider changing this later.

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.widgets.controlgraph", line 62, in mouseReleaseEvent
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.widgets.controlgraph", line 680, in updateWidget
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.widgets.axis", line 1172, in updateControlItem
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.document.doc", line 182, in applyOperation
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.document.operations", line 1064, in do
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.document.operations", line 73, in do
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.setting.setting", line 132, in set
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.setting.setting", line 588, in convertTo
InvalidType