#110 Crash when adding error bars with empty columns

[DerManu]

May seem like an artificial situation but I encountered this, so I'll just post it :)
Reproduce like this:
- Start Scidavis 0.2.2
- leave the table empty and select the y column
- plot -> line (or scatter...)
- add a function to the plot
- add error bars to the plot
=> Crash with read exception

[Knut Franke]

There may be other, less artificial ways to trigger the same bug. Since a crash can lead to severe data loss, I'm increasing the priority.