Fix container.Items nil ptr panic with some non-AWS impls of S3

[tarndt]

During testing using stow with some non-AWS S3 implementation's stow's container.Items panics due to a nil pointer dereference. This PR checks to ensure the said pointer object.StorageClass *string is not nil before dereferencing.

Fix:

Panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb3801e]
....
panic({0xc3e3a0, 0x1395120})
	~/go/src/runtime/panic.go:1038 +0x215
github.com/graymeta/stow/s3.(*container).Items(0xc0001fc100, {0xc0007a86f0, 0x14}, {0x0, 0x0}, 0x41)
	~/go/src/github.com/graymeta/stow/s3/container.go:63 +0x27e