Coqtop: fix looping over a broken state.
Coqtop: fix looping over a broken state.

Coqtop is not able to deal with broken states, hence Stm.interp
has to immediately backtrack to the last meaningful state.

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@16894 85f007b7-540e-0410-9357-904b9bb8a0f7

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