Fixed .summary() IndexError

Fixes the following observed in IDA 6.4.130111:

Python>mipsrop.summary()
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/ida/bin/plugins/mipsrop.py", line 552, in summary
    if mnem[0].lower() in ['j', 'b']:
IndexError: string index out of range