LazyChunkReader.ReadAt panics if off>0

[gbalint]

LazyChunkReader.ReadAt does not work well if off>0
E.g. if I call it with 1 for a reader on a chunk with length 16777216 it panics:

panic: runtime error: slice bounds out of range

goroutine 279475 [running]:
github.com/ethereum/go-ethereum/swarm/storage.(*LazyChunkReader).join.func1(0xc4200ba480, 0x408, 0x408, 0xc420211340, 0xc4202acc60, 0x1, 0xc4200274f8, 0xc4202acba0, 0xc425f279b0, 0xc423caa000, ...)
	/Users/gbalint/go/src/github.com/ethereum/go-ethereum/swarm/storage/chunker.go:517 +0x3c9
created by github.com/ethereum/go-ethereum/swarm/storage.(*LazyChunkReader).join
	/Users/gbalint/go/src/github.com/ethereum/go-ethereum/swarm/storage/chunker.go:516 +0x2f3
FAIL	github.com/ethereum/go-ethereum/swarm/storage	2.212s

Strongly connected to this issue: #344

[nonsense]

I got this on the test cluster as well, so it is not just a problem within tests.

swarm_8505 panic: runtime error: slice bounds out of range
swarm_8505                                                                                                                                                                   swarm_8505 goroutine 2830 [running]:
swarm_8505 github.com/ethereum/go-ethereum/swarm/storage.(*LazyChunkReader).join.func1(0xc420123b00, 0x8a8, 0x8a8, 0xc421ca6ee0, 0xc42164d380, 0x0, 0xc421b843a8, 0xc42164d2c0, 0xc421b84688, 0xc422dee000, ...)
swarm_8505      /go/src/github.com/ethereum/go-ethereum/swarm/storage/chunker.go:519 +0x52e                                                                                  swarm_8505 created by github.com/ethereum/go-ethereum/swarm/storage.(*LazyChunkReader).join
swarm_8505      /go/src/github.com/ethereum/go-ethereum/swarm/storage/chunker.go:518 +0x2f3

Difference within the line numbers is due to tracing I've added.