Deleting a sheet referenced in current sheet causes sc-im to crash.

[earthshrink]

Sc-im is able to parse out non-existent sheet at time of cell entry. If a sheet referenced in the current sheet is deleted, however, it crashes. Here's how to reproduce.

Create a test spreadsheet, say test.sc with the following lines

newsheet "Sheet2"
newsheet "Sheet1"
movetosheet "Sheet2"
let A0 = 1
goto A0
movetosheet "Sheet1"
let A0 = {"Sheet2"}!A0
goto A0

Bring up sc-im test.sc. It will show the value 1.0 in cell A0.
Enter command :delsheet "Sheet2"
Sc-im immediately segfaults.

If instead command ={"Sheet3"}!a0 is entered, referencing a non-existent sheet. it is silently rejected and cell value is set to 0.

The crash happens while the current sheet is being repainted. It doesn't crash if the detetion command is given when sc-im is run with --nocurses command option.