Weaken the external call case of safeN
in order to fill a hole in semax_call_external.
Merge branch 'master' of https://github.com/PrincetonUniversity/VST
Fill in half a hole in semax_call_external
The other half may require a strengthening of pures_sub (adding the backward direction).
Merge branch 'master' of https://github.com/PrincetonUniversity/VST