Fix crash when no secondary anchors found. #639

Job	Run time
Rust check	21s
Rust format	21s
Rust check with default features	25s
Rust test	1m 1s
Rust check (macos-latest)	1m 3s
Rust doc	1m 42s
Rust check (windows-latest)	1m 2s
Rust lint (stable)	23s
Rust lint (nightly)	23s
Rust lint (beta)	22s
Rust lint	23s
	7m 26s

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