message: Fixed build error under Android NDK
  Otherwise clang will complain about
  ```
  cannot initialize a member subobject of type 'int' with an rvalue of type 'nullptr_t'
  struct sigaction sa = {nullptr};
  ^~~~~~~
  ```
diff: @@ -1430,7 +1430,7 @@ namespace {
  sigStack.ss_size = sizeof(altStackMem);
  sigStack.ss_flags = 0;
  sigaltstack(&sigStack, &oldSigStack);
  - struct sigaction sa = {nullptr};
  + struct sigaction sa = {0};
  sa.sa_handler = handleSignal; // NOLINT
  sa.sa_flags = SA_ONSTACK;
  