CGraphBackend: fix a segfault when given a non-existing vertex #22371
Description
sage: graphs.KrackhardtKiteGraph().eccentricity("a")
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Unhandled SIGSEGV: A segmentation fault occurred.
This probably occurred because a *compiled* module has a bug
in it and is not properly wrapped with sig_on(), sig_off().
Python will now terminate.
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CC: @mezzarobba
Component: graph theory
Author: Marc Mezzarobba
Branch/Commit: fd58651
Reviewer: Jeroen Demeyer
Issue created by migration from https://trac.sagemath.org/ticket/22371