[perf2bolt] Fix for SKL bug workaround

Summary:
Some LBR traces contain less than 16/32 entries. When the first trace is
less than the standard length, we fail to enable SKL bug workaround with
BAT mode enabled. The result is a bad profile from perf2bolt. The fix
is to check the length of every trace, not just the first one.

Issue facebookincubator/BOLT#94

(cherry picked from FBD22917971)