Fix panic when Message::author_permissions cannot find channel #1205

Job	Run time
Rustfmt	9s
Clippy	53s
Test (MSRV)	1m 33s
Build docs	2m 19s
Test (stable)	1m 57s
Test (beta)	2m 0s
Test (nightly)	1m 44s
Test (macOS)	1m 42s
Test (Windows)	4m 18s
Test (no features)	1m 8s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 14s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 29s
	20m 26s

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