internal/ethapi: initialize account mutex in lock properly

internal/ethapi/api.go

@@ -212,6 +212,7 @@ type PrivateAccountAPI struct {
 func NewPrivateAccountAPI(b Backend, nonceLock *AddrLocker) *PrivateAccountAPI {
 	return &PrivateAccountAPI{
 		am:        b.AccountManager(),
+		nonceLock: nonceLock,
 		b:         b,
 	}
 }