lib/gshadow.c: endsgent(): Invert logic to reduce indentation

[alejandro-colomar]

This is in preparation for other patches.

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Revisions:

v1b

- Rebase ``` $ git range-diff master..gh/inv shadow/master..inv 1: c8799756 = 1: 98bc1d51 lib/gshadow.c: endsgent(): Invert logic to reduce indentation ```

v2

- Several rebases ``` $ git range-diff 98bc1d51^..98bc1d51 shadow/master..gh/inv 1: 98bc1d51 = 1: 5efa45dc lib/gshadow.c: endsgent(): Invert logic to reduce indentation ```

[alejandro-colomar]

@ikerexxe, @hallyn The CI failures are due to the broken github runners. Feel free to review and merge.