[BOLT] Always call fixBranches in relocation mode.

Summary:
If you attempted to use a function filter on a binary when in relocation mode, the resulting binary would probably crash.  This is because we weren't calling fixBranches on all functions.  This was breaking bughunter.sh

I also strengthened the validation of basic blocks.  The cond branch should always be non-null when there are two successors.

(cherry picked from FBD6261930)