repopick: Don't crash if change not found

If change is not located, skip it. Resolves the following:

Traceback (most recent call last):
  File "(omitted)/build/tools/repopick.py", line 258,
in <module>
    review = [x for x in reviews if x['number'] == change][0]
IndexError: list index out of range

Change-Id: I96423ddc809ac1c7994998db4e1929ca813f20ca