STR$ format code for negative format may not work right

[rlamorea]

By code inspection, it really looks like the parsing of the negative format is messed up.

For one thing, the code that looks correct is assigned to C -- and that is the second C case
in the switch, which means it won't be hit.

The code for S which appears to be setting negative format, looks like incomplete code.

Finally, it seems like N would be a better format code.