i#1898 legacy child: avoid crash and warn of missing child
i#1898 legacy child: avoid crash and warn of missing child

Avoids a crash for a legacy NtCreateProcess{,Ex} call on Vista+ by aborting
on the attempt to inject at process creation time with no context and early
injection disabled.  Issues a warning, as we may miss the child -- which
seems to be the case in certain instances on Windows 10 which need further
investigation.

Review-URL: https://codereview.appspot.com/292820043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio