shortest_path() segfaults if vertex not present

Bug reports:

Please follow the general troubleshooting steps first:

• Are you running the latest graph-tool version?
• Do you observe the problem with the current git version?
• Are you using Macports or Homebrew? If yes, please submit an issue there instead: https://github.com/Homebrew/brew/issues and https://trac.macports.org/newticket
• Did you compile graph-tool manually?

Do not forget to supply the following information:

• A minimal and self-contained example that shows the problem.
• Your operating system.
• The Python version you are using.
• If you compiled graph-tool manually: Your compiler version, as well as the version of Boost being used.

Setup

• gt.__version__ == 2.43 (commit 5778eb10, )
• Python version: 3.9.7
• uname -a Linux hostname 5.11.0-38-generic #42~20.04.1-Ubuntu SMP Tue Sep 28 20:41:07 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• Installed / automatically compiled via conda-forge

MWE

import graph_tool.all as gt
gt.shortest_path(gt.Graph(), 1, 2)
Segmentation fault (core dumped)

Might be related to #712 (closed)