sign: check if the http request body is empty

Commands using an empty request body will panic:

$ ./glacier vault list us-east-1
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x44e6fd]

goroutine 1 [running]:
io/ioutil.func·001()
        /home/msantos/src/3rd/go/src/pkg/io/ioutil/ioutil.go:29 +0xee
bytes.(*Buffer).ReadFrom(0xc2000c8380, 0x0, 0x0, 0x0, 0x0, ...)
        /home/msantos/src/3rd/go/src/pkg/bytes/buffer.go:169 +0x1fd
io/ioutil.readAll(0x0, 0x0, 0x200, 0x0, 0x0, ...)
        /home/msantos/src/3rd/go/src/pkg/io/ioutil/ioutil.go:32 +0x143
io/ioutil.ReadAll(0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/msantos/src/3rd/go/src/pkg/io/ioutil/ioutil.go:41 +0x3d
github.com/rdwilliamson/aws.(*Signature).Sign(0xc2000d00c0, 0xc2000b5680, 0x0, 0x0, 0xa, ...)
        /home/msantos/src/golang/gocode/src/github.com/rdwilliamson/aws/aws.go:307 +0xf3d
github.com/rdwilliamson/aws/glacier.(*Connection).ListVaults(0xc2000de300, 0x6cb1a0, 0x0, 0x0, 0x0, ...)
        /home/msantos/src/golang/gocode/src/github.com/rdwilliamson/aws/glacier/vault.go:206 +0x2a5
main.vault(0xc200092040, 0x1, 0x1)
        /home/msantos/src/golang/gocode/src/github.com/rdwilliamson/glacier/vault.go:53 +0x9c8
main.main()
        /home/msantos/src/golang/gocode/src/github.com/rdwilliamson/glacier/main.go:82 +0x89f

goroutine 2 [syscall]:

goroutine 3 [runnable]: