clear_edges() and clear() do not reset the number of edges.

Bug reports:

clear_edges() and clear() do not reset the number of edges.

I tested with an environment below.
OS: macOS Sierra 10.12.5
Python version: 2.7.13
graph-tool version: 2.22_2

An example code producing the problem is below:

from graph_tool.all import *

# Case 1
g = Graph(directed=True)
e = g.add_edge(0, 1)

g.clear_edges()
print(g.num_edges())  # 1

g.remove_edge(e)
print(g.num_edges())  # 1

g.clear()
print(g.num_edges())  # 1

# Case 2
g = Graph(directed=True)
e = g.add_edge(0, 1)

g.remove_edge(e)
print(g.num_edges())  # 0

g.clear_edges()
print(g.num_edges())  # 0