`_index_vertex_edges` used truthiness checks (`if not edge.source_vertex_index`)
to skip edges with absent fields, but this also silently drops any edge whose
source or target is vertex index 0 — a valid vertex. Both fields are protobuf
optional integers, so the correct absent-field check is `HasField()`, consistent
with `_index_flow_graph_edges` and `_index_call_graph_vertices` in the same file.
In mimikatz.exe_.ida.BinExport, vertex 0 at 0x401000 has 2 callees and 1 caller
that were all being silently discarded.
Willi Ballenthin
47d418b7de