Don't loop to your self in synthesized Begin records.

This would cause the API to link this log record to itself, creating
endless loops. Correct it by setting it to zero (meaning no parent).

lib/libvarnishapi/vsl_dispatch.c

@@ -881,7 +881,7 @@ vtx_force(struct VSLQ *vslq, struct vtx *vtx, const char *reason)
 	vtx_scan(vslq, vtx);
 	if (!(vtx->flags & VTX_F_BEGIN))
 		vtx_synth_rec(vtx, SLT_Begin, "%s %u synth",
-		    vsl_t_names[vtx->type], vtx->key.vxid);
+		    vsl_t_names[vtx->type], 0);
 	vtx_diag(vtx, reason);
 	if (!(vtx->flags & VTX_F_END))
 		vtx_synth_rec(vtx, SLT_End, "synth");