Fix uninitialized fields in grompp t_state

Fix uninitialized fields in grompp t_state

Allocating t_state on the stack and using an incomplete
pseudo-constructor means it is possible to write an uninitialized
value in fep_state field to the .tpr file, which we've been
doing. Found with Memory Sanitizer. Perhaps this behaviour lies behind
some of the strange behaviour that is periodically seen on Jenkins.

Fixed by allocating t_state on the heap, which might resolve other
issues, since snew() zeroes the memory as a side effect. Also
initialized fep_state field in init_state().

Refs #1729, #1730

Change-Id: Ibcee7bff1e090fb1991969c4562f44f056868a03