[m-dev.] diff: `do_runnext'-related bug fixes

[Fergus Henderson]

Oops, I omitted the summary from the log message for that diff.
The summary was:

	Fix some bugs relating to `do_runnext' that caused things to fail
	in grades `jump' and `fast'.

The bugs caused the test cases on kryten to fail.

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Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
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