Fix a crash happening in NBLog's 'writeHistToFile'

The issue was in the possibility of the 'intervals' vector being
empty if the provided 'samples' vector contains only one
element. That leads to 'intervals.end()' being passed to
'intervals.erase()', and vector::erase produces undefined
behaviour in that case.

I see no other calls to 'erase' or 'max_element' in libnbaio,
so that should be the only fix needed.

Bug: 62536501
Test: see test case at bug
Change-Id: I37f93fa42030ed6f5bcb2d46b0a7a2c0af3c82e4