[m-rev.] for post-commit review: fix mantis bug 50

[Zoltan Somogyi]

For review by anyone.

This bug was tough to track down, because it manifested itself
only in the presence of several rare circumstances.

While the fix is simpler than the complexity of the history
that gives rise to the bug (which is described in tests/valid/
bug50.m), I couldn't summarize it in one line for the log message.

Zoltan.
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