fix(scyllaclient): don't panic on empty host in client

In case client was mistakenly initialized with "" host, getting host from the pool would panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x11da5ac]
goroutine 253 [running]:
github.com/scylladb/scylla-manager/v3/pkg/scyllaclient.NewClient.hostPool.func3(0xc0004345a0)
github.com/scylladb/scylla-manager/v3/pkg/scyllaclient/hostpool.go:32 +0xac

This should be replaced with a simple error.
This commit adds additional validation to client config,
which prohibits specifying empty host there.
It also makes host pool return an error instead of panic
in case of invalid host pool response.

Fixes scylladb/scylla-enterprise#4692

pkg/scyllaclient/config.go

@@ -4,6 +4,7 @@ package scyllaclient
 
 import (
 	"net/http"
+	"slices"
 	"time"
 
 	"github.com/pkg/errors"
@@ -111,6 +112,9 @@ func (c Config) Validate() error {
 	if len(c.Hosts) == 0 {
 		err = multierr.Append(err, errors.New("missing hosts"))
 	}
+	if slices.Contains(c.Hosts, "") {
+		err = multierr.Append(err, errors.New("empty host"))
+	}
 	if c.Port == "" {
 		err = multierr.Append(err, errors.New("missing port"))
 	}

pkg/scyllaclient/hostpool.go

@@ -29,6 +29,9 @@ func hostPool(next http.RoundTripper, pool hostpool.HostPool, port string) http.
 		// Get host from pool
 		if !ok {
 			hpr = pool.Get()
+			if hpr == nil {
+				return nil, errors.New("empty host pool response")
+			}
 			h = hpr.Host()
 		}