health: do not rely on the operational DS being valid

I've seen the following crash on the real HW recently:

  #0  0x00000000b6c7979c _ZNK7libyang8DataNode6isTermEv (libyang-cpp.so + 0x1b79c)
  #1  0x00000000b6c797e8 _ZNK7libyang8DataNode6asTermEv (libyang-cpp.so + 0x1b7e8)
  #2  0x00000000004927ec operator()<unsigned int, std::basic_string_view<char>, std::optional<std::basic_string_view<char> >, sysrepo::Event, unsigned int> (veliad-health + 0xc7ec)
  #3  0x00000000b6da378c _ZNKSt8functionIFN7sysrepo9ErrorCodeENS0_7SessionEjSt17basic_string_viewIcSt11char_traitsIcEESt8optionalIS6_ENS0_5EventEjEEclES2_jS6_S8_S9_j (libsysrepo-cpp.so + 0x1378c)
  #4  0x00000000b6d1b5a0 sr_module_change_subscribe_enable (libsysrepo.so.7 + 0x135a0)
  #5  0x00000000b6da451c _ZN7sysrepo12Subscription14onModuleChangeERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt8functionIFNS_9ErrorCodeENS_7SessionEjSt17basic_string_viewIcS4_ESt8optionalISD_ENS_5EventEjEERKSE_IS6_EjNS_16SubscribeOptionsE (libsysrepo-cpp.so + 0x1451c)
  #6  0x00000000b6d9ad04 _ZN7sysrepo7Session14onModuleChangeERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt8functionIFNS_9ErrorCodeES0_jSt17basic_string_viewIcS4_ESt8optionalISC_ENS_5EventEjEERKSD_IS6_EjNS_16SubscribeOptionsES9_IFvRSt9exceptionEERKSD_INS_10FDHandlingEE (libsysrepo-cpp.so + 0xad04)
  #7  0x0000000000493990 _ZN5velia6health13AlarmsOutputsC2EN7sysrepo7SessionERKSt6vectorISt8functionIFvNS0_5StateEEESaIS8_EE (veliad-health + 0xd990)
  #8  0x000000000048e214 main (veliad-health + 0x8214)
  #9  0x00000000b683e8a8 n/a (libc.so.6 + 0x228a8)
  ELF object binary architecture: ARM

...which happened while I was playing with some service restarts. Since
I do not have anything more complete, my working theory is that
sysrepo-ietf-alarms was dead/dying at the time this code run, and once
that happened, this callback was invoked with information about a
removal of the alarm-summary data. That sounds like something which can
happen, and once that happens, the original code would segfault because
it dereferenced an empty std::optional.

Fix that by wrapping stuff in an if.

Change-Id: If9365fa47f09f1cc0e6cb37a2b149c7a587010c3

src/health/outputs/AlarmsOutputs.cpp

@@ -56,8 +56,14 @@ AlarmsOutputs::AlarmsOutputs(sysrepo::Session session, const std::vector<std::fu
 
             if (auto data = session.getData(alarmSummary)) {
                 for (const auto& [severity, errorState] : severityToHealthStateMapping) {
-                    const auto activeAlarms = std::stoi(std::string(data->findPath(alarmSummary + "/alarm-summary[severity='"s + severity + "']/not-cleared")->asTerm().valueStr()));
+                    const auto node = data->findPath(alarmSummary + "/alarm-summary[severity='"s + severity + "']/not-cleared");
+                    if (!node) {
+                        // we are not getting any data, surely that's bad
+                        state = State::ERROR;
+                        break;
+                    }
 
+                    const auto activeAlarms = std::stoi(std::string(node->asTerm().valueStr()));
                     if (activeAlarms > 0) {
                         state = errorState;
                         break;