Heads: avoid potentially uncaught Not_found via an assert false
Heads: avoid potentially uncaught Not_found via an assert false

 The underlying issue (#2608) should be fixed now. In case something
 similar happens again, we won't be chasing again a Not_found here.
 Same idea as commit r14550, but this initial attempt was too zealous:
 we shouldn't also "protect" variable_head, since a Not_found in it is
 handled a few lines later.

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@14588 85f007b7-540e-0410-9357-904b9bb8a0f7

maximedenes/native-coq,maximedenes/native-coq,maximedenes/native-coq