Fail pony_start if ASIO backend wasn't successfully initialized

Prior to this commit, the ASIO backend could fail initialization
but the runtime would continue to start up anyway resulting in
segfaults or assertion failures.

This commit changes things so that if the backend initialization
fails, the runtime exits immediately with an error.

This commit also fixes the asio restart logic from PR #2373 to
properly re-initialize the backend first.

src/libponyrt/asio/asio.c

@@ -46,6 +46,10 @@ void ponyint_asio_init(uint32_t cpu)
 
 bool ponyint_asio_start()
 {
+  // if the backend wasn't successfully initialized
+  if(running_base.backend == NULL)
+    return false;
+
   if(!ponyint_thread_create(&running_base.tid, ponyint_asio_backend_dispatch,
     asio_cpu, running_base.backend))
     return false;

src/libponyrt/sched/scheduler.c

@@ -29,6 +29,7 @@ typedef enum
 } sched_msg_t;
 
 // Scheduler global data.
+static uint32_t asio_cpu;
 static uint32_t scheduler_count;
 static scheduler_t* scheduler;
 static PONY_ATOMIC(bool) detect_quiescence;
@@ -113,10 +114,8 @@ static bool read_msg(scheduler_t* sched)
         if (sched->asio_stopped)
         {
           // restart the ASIO thread
-          bool asio_started = ponyint_asio_start();
-          pony_assert(asio_started);
-          (void) asio_started;
-          sched->asio_stopped = false;
+          ponyint_asio_init(asio_cpu);
+          sched->asio_stopped = !ponyint_asio_start();
         }
 
         // make sure asio hasn't already been stopped or else runtime is in
@@ -470,7 +469,7 @@ pony_ctx_t* ponyint_sched_init(uint32_t threads, bool noyield, bool nopin,
     scheduler_count * sizeof(scheduler_t));
   memset(scheduler, 0, scheduler_count * sizeof(scheduler_t));
 
-  uint32_t asio_cpu = ponyint_cpu_assign(scheduler_count, scheduler, nopin,
+  asio_cpu = ponyint_cpu_assign(scheduler_count, scheduler, nopin,
     pinasio);
 
   for(uint32_t i = 0; i < scheduler_count; i++)