Buffer fields hit assertion when applied to unnamed buffers

[avdgrinten]

When a buffer field (BITFIELD_OP, BYTEFIELD_OP, WORDFIELD_OP, DWORDFIELD_OP, QWORDFIELD_OP or ARBFIELD_OP) targets an unnamed buffer (e.g., an immediate operand), LAI dies with the error below.

This should be fixed by storing a reference to the buffer in struct lai_nsnodes of type LAI_NAMESPACE_BUFFER_FIELD (and not a reference to another struct lai_nsnode as we do currently). More precisely, we should store a struct lai_buffer_head * (and increment its reference count using lai_rc_ref).