Fix panic in QueryNextRoomHierarchyPage (#3253)
Sentry reported the following panic:
```
time="2023-11-01T01:33:56.220583478Z" level=error msg="Request panicked!
goroutine 43763845 [running]:
runtime/debug.Stack()
runtime/debug/stack.go:24 +0x5e
github.com/matrix-org/dendrite/internal/httputil.MakeExternalAPI.MakeJSONAPI.Protect.func3.1()
github.com/matrix-org/util@v0.0.0-20221111132719-399730281e66/json.go:98 +0x13e
panic({0x15b5540?, 0x2453560?})
runtime/panic.go:914 +0x21f
github.com/matrix-org/dendrite/internal/httputil.MakeAuthAPI.func1.1()
github.com/matrix-org/dendrite/internal/httputil/httpapi.go:91 +0x4a
panic({0x15b5540?, 0x2453560?})
runtime/panic.go:914 +0x21f
github.com/matrix-org/dendrite/roomserver/internal/query.(*Queryer).QueryNextRoomHierarchyPage(0x413185?, {0x1a576e0, 0xc0436705a0}, {{{0xc01e5fd260, 0x1f}, {0xc01e5fd261, 0x12}, {0xc01e5fd274, 0xb}}, {0xc145cb5200, ...}, ...}, ...)
github.com/matrix-org/dendrite/roomserver/internal/query/query_room_hierarchy.go:116 +0xbfe
github.com/matrix-org/dendrite/clientapi/routing.QueryRoomHierarchy(0xc0be13b200, 0xc144e65dd0, {0xc01e5fd260?, 0x6?}, {0x7faf140639c8, 0xc00059af20}, 0xc08adca000?)
github.com/matrix-org/dendrite/clientapi/routing/room_hierarchy.go:141 +0x68b
github.com/matrix-org/dendrite/clientapi/routing.Setup.func35(0xc03e7d5c20?, 0x17c3a57?)
github.com/matrix-org/dendrite/clientapi/routing/routing.go:534 +0xbe
github.com/matrix-org/dendrite/internal/httputil.MakeAuthAPI.func1(0xc0bd097300)
github.com/matrix-org/dendrite/internal/httputil/httpapi.go:108 +0x5ed
github.com/matrix-org/util.(*jsonRequestHandlerWrapper).OnIncomingRequest(0xc0bd097200?, 0xc13b7d6fc0?)
github.com/matrix-org/util@v0.0.0-20221111132719-399730281e66/json.go:79 +0x19
github.com/matrix-org/dendrite/internal/httputil.MakeExternalAPI.MakeJSONAPI.func2({0x1a54880, 0xc138f28b60}, 0xc0bd097200?)
github.com/matrix-org/util@v0.0.0-20221111132719-399730281e66/json.go:141 +0xaa
github.com/matrix-org/dendrite/internal/httputil.MakeExternalAPI.MakeJSONAPI.Protect.func3({0x1a54880?, 0xc138f28b60?}, 0x17c01d9?)
github.com/matrix-org/util@v0.0.0-20221111132719-399730281e66/json.go:103 +0x63
net/http.HandlerFunc.ServeHTTP(...)
net/http/server.go:2136
github.com/matrix-org/dendrite/internal/httputil.MakeExternalAPI.func1({0x1a54880?, 0xc138f28b60?}, 0xc0bd097100)
github.com/matrix-org/dendrite/internal/httputil/httpapi.go:191 +0x411
net/http.HandlerFunc.ServeHTTP(0xc0bd097000?, {0x1a54880?, 0xc138f28b60?}, 0xbe1348905308878e?)
net/http/server.go:2136 +0x29
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000000000, {0x1a54880, 0xc138f28b60}, 0xc0bd096f00)
github.com/gorilla/mux@v1.8.0/mux.go:210 +0x1c5
github.com/matrix-org/dendrite/setup/base.SetupAndServeHTTP.(*Handler).Handle.(*Handler).handle.func5({0x1a54880, 0xc138f28b60}, 0xc0bd096e00)
github.com/getsentry/sentry-go@v0.14.0/http/sentryhttp.go:103 +0x298
net/http.HandlerFunc.ServeHTTP(0xc0bd096a00?, {0x1a54880?, 0xc138f28b60?}, 0x7fae6812f5d0?)
net/http/server.go:2136 +0x29
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000000a80, {0x1a54880, 0xc138f28b60}, 0xc0bd096900)
github.com/gorilla/mux@v1.8.0/mux.go:210 +0x1c5
net/http.serverHandler.ServeHTTP({0xc02884c4e0?}, {0x1a54880?, 0xc138f28b60?}, 0x6?)
net/http/server.go:2938 +0x8e
net/http.(*conn).serve(0xc1926922d0, {0x1a576e0, 0xc024a6ec90})
net/http/server.go:2009 +0x5f4
created by net/http.(*Server).Serve in goroutine 16979
net/http/server.go:3086 +0x5cb
" context=missing panic="runtime error: invalid memory address or nil pointer dereference"
```
[skip ci]
5f872f4a82