Fixes crash cleaning media

- Unsure of how it’s possible, but added a nil check
Stats views/visitor basic chart
Merge branch 'feature/986-stats' of github.com:wordpress-mobile/WordPress-iOS into feature/986-stats

Conflicts:
	WordPress/Classes/StatsViewController.m
	WordPress/Classes/StatsViewsVisitorsBarGraphCell.h
	WordPress/WordPress.xcodeproj/project.pbxproj