[CRASH] fix gh-2976. In rare condition with node with outputs not used, some optimization combination could cause crash in the fgraph integrety. fgraph.variables should contain all variables, even if they aren't used.

Remove assert in internal function. They are only called by other internal fct and will fail later anyway. expect small speed up.

Add breakpoint Op to Theano

[CRASH] Subtensor list

Do not decref descriptor, as PyArray_View steals the reference

[CRASH,TESTS] Fix crash recently introduced and fix tests

Add MRG_RandomStreams.seed

Scan test values

I could not find a way of testing that code, since scalars
are not well-supported by scan.