multi: fix recent govet findings

[dajohi]

blockmanager.go:736:14: nilness: tautological condition: non-nil != nil (govet)
        if template != nil {
                    ^
mining.go:854:19: nilness: tautological condition: nil == nil (govet)
                        if curTemplate == nil {
                                       ^
rpcserver.go:2949:22: nilness: impossible condition: non-nil == nil (govet)
        if expectedPrevHash == nil || !expectedPrevHash.IsEqual(prevHash) {
                            ^
rpcserver.go:3236:41: nilness: nil dereference in dynamic method call (govet)
                return nil, rpcInternalError(err.Error(),
                                                      ^
rpcserver.go:4101:15: nilness: impossible condition: non-nil == nil (govet)
                if msgBlock == nil {
                            ^

[davecgh]

This is currently failing Travis.