Fix nil pointer derefernce issues (#3379)

Discovered while running
https://gitlab.futo.org/load-testing/matrix-goose.

Dendrite locks up and runs into `context cancelled`, so the error is not
`sql.ErrNoRows` nor "default" (and definitely shouldn't return that the
account exists in this case)

roomserver/storage/postgres/user_room_keys_table.go

@@ -162,6 +162,9 @@ func (s *userRoomKeysStatements) SelectAllPublicKeysForUser(ctx context.Context,
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
+	if err != nil {
+		return nil, err
+	}
 	defer internal.CloseAndLogIfError(ctx, rows, "SelectAllPublicKeysForUser: failed to close rows")
 
 	resultMap := make(map[types.RoomNID]ed25519.PublicKey)

roomserver/storage/sqlite3/user_room_keys_table.go

@@ -177,6 +177,9 @@ func (s *userRoomKeysStatements) SelectAllPublicKeysForUser(ctx context.Context,
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
+	if err != nil {
+		return nil, err
+	}
 	defer internal.CloseAndLogIfError(ctx, rows, "SelectAllPublicKeysForUser: failed to close rows")
 
 	resultMap := make(map[types.RoomNID]ed25519.PublicKey)

userapi/internal/user_api.go

@@ -939,11 +939,12 @@ func (a *UserInternalAPI) QueryAccountByPassword(ctx context.Context, req *api.Q
 		return nil
 	case bcrypt.ErrHashTooShort: // user exists, but probably a passwordless account
 		return nil
-	default:
+	case nil:
 		res.Exists = true
 		res.Account = acc
 		return nil
 	}
+	return err
 }
 
 func (a *UserInternalAPI) SetDisplayName(ctx context.Context, localpart string, serverName spec.ServerName, displayName string) (*authtypes.Profile, bool, error) {