Using `pss_sendRaw` with address `0x` crashes the node

[PaulLeCam]

System information

Commit hash: 09d588e0da77c1ed150d7274f66a67dcaecab80c (latest master at the time)

Expected behaviour

Calling pss_sendRaw with the 0x value for the address would be supported.

Actual behaviour

The node trying to send the message crashes.

Steps to reproduce the behaviour

Call pss_sendRaw with 0x as the first argument.

Backtrace

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x472abb8]

goroutine 620 [running]:
github.com/ethereum/go-ethereum/swarm/pss.(*Pss).SendRaw(0xc0001be420, 0x57b13c0, 0x0, 0x0, 0xc0af3bd7cf, 0xc00024e560, 0xb, 0xb, 0xc012672801, 0xc0126728c0)
	/Users/paul/go/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:773 +0x298
github.com/ethereum/go-ethereum/swarm/pss.(*API).SendRaw(0xc00000fc08, 0x57b13c0, 0x0, 0x0, 0xaf3bd7cf, 0xc00024e560, 0xb, 0xb, 0x0, 0x0)
	/Users/paul/go/src/github.com/ethereum/go-ethereum/swarm/pss/api.go:168 +0x82
reflect.Value.call(0xc011050580, 0xc011046c48, 0x13, 0x4b6ba64, 0x4, 0xc0127e8960, 0x4, 0x4, 0xc012188100, 0xc0127e8960, ...)
	/usr/local/Cellar/go/1.11.2/libexec/src/reflect/value.go:447 +0x449
reflect.Value.Call(0xc011050580, 0xc011046c48, 0x13, 0xc0127e8960, 0x4, 0x4, 0x3, 0x3, 0xc0000b0748)
	/usr/local/Cellar/go/1.11.2/libexec/src/reflect/value.go:308 +0xa4
github.com/ethereum/go-ethereum/rpc.(*Server).handle(0xc011080990, 0x4ce2260, 0xc0126da210, 0x4ce7340, 0xc0127d4a50, 0xc0127e8420, 0xc000173738, 0x402d832, 0xc000000008)
	/Users/paul/go/src/github.com/ethereum/go-ethereum/rpc/server.go:309 +0x6c3
github.com/ethereum/go-ethereum/rpc.(*Server).exec(0xc011080990, 0x4ce2260, 0xc0126da210, 0x4ce7340, 0xc0127d4a50, 0xc0127e8420)
	/Users/paul/go/src/github.com/ethereum/go-ethereum/rpc/server.go:330 +0x1a6
github.com/ethereum/go-ethereum/rpc.(*Server).serveRequest.func2(0xc00024e1b0, 0xc011080990, 0xc000396900, 0x4ce7340, 0xc0127d4a50, 0xc011046cd8, 0x1, 0x1, 0x5f33000)
	/Users/paul/go/src/github.com/ethereum/go-ethereum/rpc/server.go:204 +0xad
created by github.com/ethereum/go-ethereum/rpc.(*Server).serveRequest
	/Users/paul/go/src/github.com/ethereum/go-ethereum/rpc/server.go:199 +0x28a