Bug in prickelpit.c function slp_cannot_execute()

[akruis]

The error handling function slp_cannot_execute() is buggy. Because this function usually does not get called, I missed it when I converted the frame functions in #117. Now I used the function and observed a crash. Fortunately, the fix is straight forward.

[akruis]

Fortunately it turned out that the bug is just a minor one and can't be triggered by released versions of Stackless. But the code in pull request #178 triggers it.

Therefore I won't backport the fix to 2.7 - 3.6.