proced: sort commands throw an error

[T.V. Raman]

Latest Git snapshot of Emacs 24:

M-x proced
fa to show all processes (filter all)
All the sort functions bomb e.g. 's c' to sort by cpu time, or
M-x proced-sort-interactive -- then specify sort criteria.

Sample backgrace buffer follows:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  proced-sort-pcpu(no-arg)
  call-interactively(proced-sort-pcpu nil nil)

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