Handle an hfindex of -1.

Code generated by the current version of Pidl dissects bitset fields
twice - once with proto_tree_add_item(), which puts the actual item into
the tree, so that we can create a subtree under it, and once with a
dissect_dcerpc_{type}() routine, to fetch the value and step past the
field.  The second call passes an hf_ index value of -1; interpret that
as meaning "don't put this into the protocol tree", rather than just
blindly calling proto_tree_add_XXX with -1 and getting a dissector
assertion.

Change-Id: I42fc023e77514a8a8a25d5c9db11fa3ef053babf
Reviewed-on: https://code.wireshark.org/review/4892
Reviewed-by: Guy Harris <guy@alum.mit.edu>