Fix extra_cmds is None crash

The previous patch introduced extra_cmds flag to the command line,
but somehow not checking if such flag was never added.
This patch add check to avoid script crashing.

Change-Id: I18071e5f082f8058c8f8fa78bfe69f7ef5ef560d
Test: lldbclient.py (without --extra-cmds-file)
1 file changed