Improve input validation for --hartids flag

Disallow negative hartids, repeated hartids, and empty lists.

spike_main/spike.cc

@@ -299,10 +299,28 @@ static std::vector<size_t> parse_hartids(const char *s)
 
   int n;
   while (stream >> n) {
+    if (n < 0) {
+      fprintf(stderr, "Negative hart ID %d is unsupported\n", n);
+      exit(-1);
+    }
+
     hartids.push_back(n);
     if (stream.peek() == ',') stream.ignore();
   }
 
+  if (hartids.empty()) {
+    fprintf(stderr, "No hart IDs specified\n");
+    exit(-1);
+  }
+
+  std::sort(hartids.begin(), hartids.end());
+
+  const auto dup = std::adjacent_find(hartids.begin(), hartids.end());
+  if (dup != hartids.end()) {
+    fprintf(stderr, "Duplicate hart ID %zu\n", *dup);
+    exit(-1);
+  }
+
   return hartids;
 }