lnwallet: fix panic on aux sig handling #15832

Job	Run time
lint code	7m 41s
check commits	5m 8s
check pinned dependencies (google.golang.org/grpc v1.59.0)	5s
cross compilation	31m 12s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	3s
RPC and mobile compilation check	5m 40s
sample configuration check	3m 50s
Sqlc check	5m 1s
run ubuntu itests (btcd, backend=btcd cover=1)	40m 18s
run windows itest	46m 23s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	50m 37s
run unit tests (btcd unit-cover)	17m 32s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	49m 34s
run unit tests (unit tags="kvdb_etcd")	18m 51s
run macOS itest	1h 3m 57s
check release notes updated	4s
run unit tests (unit tags="kvdb_postgres")	18m 3s
run unit tests (unit tags="kvdb_sqlite")	16m 29s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	51m 28s
run unit tests (btcd unit-race)	35m 24s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	1h 10m 46s
run unit tests (unit-module)	4m 17s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 27m 36s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	53m 10s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	1h 27m 49s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	54m 3s
run ubuntu itests (neutrino, backend=neutrino cover=1)	50m 48s
finish	8s
	14h 35m 57s

Provide feedback