Fix NPE on adding access control for review

When adding an access control for review on a completely new
project with no base, we get a NullPointerException. This is
not the case for direct changes. Moved out the code for finding
base to a function to be called in both cases.

Change-Id: Ia0453d4f5bef322c2793ce2144671cb09b4d3691
1 file changed