slic: fss, fix store to ExtraRef

We store to zero offset, not to a variable.
slic: handle memset/memcpy correctly with field-sensitive pts-to

Both memcpy and memset DEFine all values from 0 to the size of
memset/memcpy (3rd argument). The same for REFerences.

We need to compute the len argument first. A helper for that is
introduced.
kleerer: proper arguments for klee_make_symbolic

The second argument is size_t not, unsigned. Fix that.
kleerer: only insert klee_make_symbolic once

* do not delete it after creation.
* do not add a new one, when there is one from previous phases.