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

@@ -104,6 +104,7 @@ AttachedProbe::AttachedProbe(Probe &probe, std::tuple<uint8_t *, uintptr_t> func
 
 AttachedProbe::~AttachedProbe()
 {
+  if (progfd_ >= 0)
     close(progfd_);
 
   int err = 0;

src/map.cpp

@@ -101,6 +101,7 @@ Map::Map(enum bpf_map_type map_type)
 
 Map::~Map()
 {
+  if (mapfd_ >= 0)
     close(mapfd_);
 }