Don't run callback if a callback is not set

speedtest_cli.py

@@ -784,6 +784,7 @@ class Speedtest(object):
                 while thread.isAlive():
                     thread.join(timeout=0.1)
                 finished.append(sum(thread.result))
+                if callback:
                     callback(thread.i, url_count, end=True)
                 del thread
 
@@ -834,6 +835,7 @@ class Speedtest(object):
                 while thread.isAlive():
                     thread.join(timeout=0.1)
                 finished.append(thread.result)
+                if callback:
                     callback(thread.i, size_count, end=True)
                 del thread