Fix heap commands when the glibc arena has not set all expected members

gef.py

@@ -750,12 +750,14 @@ def __init__(self, addr, name=None):
  except:
  self.__arena = MallocStateStruct(addr)
  self.__addr = self.__arena.addr
- finally:
+ try:
  self.top = int(self.top)
  self.last_remainder = int(self.last_remainder)
  self.n = int(self.next)
  self.nfree = int(self.next_free)
  self.sysmem = int(self.system_mem)
+ except gdb.error as e:
+ err("Glibc arena: {}".format(e))
  return
 
  def __getitem__(self, item):