pallet-migrations: fix index access for singluar migrations #729

Job	Run time
cargo-check-all-crate-macos	9m 14s
set-image	9s
test-frame-ui	31m 6s
test-full-crypto-feature	2m 30s
cargo-check-benches (master)	24m 36s
cargo-check-each-crate (1)	17m 24s
cargo-check-benches (current)	27m 39s
cargo-check-each-crate (2)	17m 56s
cargo-check-each-crate (3)	5m 10s
test-node-metrics	8m 22s
cargo-check-each-crate (4)	14m 40s
cargo-check-each-crate (5)	21m 42s
cargo-check-each-crate (6)	13m 17s
cargo-check-each-crate (7)	27m 42s
test-frame-examples-compile-to-wasm	2m 57s
check-tracing	2m 12s
test-deterministic-wasm	6m 29s
check-metadata-hash	5m 46s
cargo-hfuzz	5m 55s
node-bench-regression-guard	13s
All test misc tests passed	1s
	4h 5m 0s

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