Apply patch: Fix crash parsing a --synth-domain with no prefix.

[DL6ER]

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Apply a patch streamlined for dnsmasq v2.80.

This problem was introduced in 2.79. This fixes #367.

This patch would also be part of dnsmasq v2.80, however, it is unclear if 2.80 will be stable before we release Pi-hole v4.1. It won't be an issue to upgrade the internal dnsmasq code to 2.80 later as we do the update by applying a series of patches where this patch will then simply be skipped with "nothing to do".

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