Fix panic when parsing sftp URIs

Closes #587

src/restic/backend/sftp/config.go

@@ -2,6 +2,7 @@ package sftp
 
 import (
 	"errors"
+	"fmt"
 	"net/url"
 	"path"
 	"strings"
@@ -31,7 +32,12 @@ func ParseConfig(s string) (interface{}, error) {
 			user = url.User.Username()
 		}
 		host = url.Host
-		dir = url.Path[1:]
+		dir = url.Path
+		if dir == "" {
+			return nil, fmt.Errorf("invalid backend %q, no directory specified", s)
+		}
+
+		dir = dir[1:]
 	case strings.HasPrefix(s, "sftp:"):
 		// parse the sftp:user@host:path format, which means we'll get
 		// "user@host:path" in s

src/restic/backend/sftp/config_test.go

@@ -74,3 +74,17 @@ func TestParseConfig(t *testing.T) {
 		}
 	}
 }
+
+var configTestsInvalid = []string{
+	"sftp://host:dir",
+}
+
+func TestParseConfigInvalid(t *testing.T) {
+	for i, test := range configTestsInvalid {
+		_, err := ParseConfig(test)
+		if err == nil {
+			t.Errorf("test %d: invalid config %s did not return an error", i, test)
+			continue
+		}
+	}
+}