Call close() only if opened

Valgrind was complaining:
==9103== Warning: invalid file descriptor -1 in syscall close()
==9103== Warning: invalid file descriptor -1 in syscall close()

src/attached_probe.cpp

@@ -104,7 +104,8 @@ AttachedProbe::AttachedProbe(Probe &probe, std::tuple<uint8_t *, uintptr_t> func
 
 AttachedProbe::~AttachedProbe()
 {
-  close(progfd_);
+  if (progfd_ >= 0)
+    close(progfd_);
 
   int err = 0;
   for (int perf_event_fd : perf_event_fds_)

src/map.cpp

@@ -101,7 +101,8 @@ Map::Map(enum bpf_map_type map_type)
 
 Map::~Map()
 {
-  close(mapfd_);
+  if (mapfd_ >= 0)
+    close(mapfd_);
 }
 
 } // namespace bpftrace