Fix errors when using multiple referenced libraries.

[wlmiller]

Addresses #8.

There's still a separate issue where referenced libraries with 0 coverage will cause errors because no HitsFile is generated for them. Adding the check in Coverage.CalculateCoverage would be one way to handle that case.

[coveralls]

Pull Request Test Coverage Report for Build #22

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 82.967%

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Files with Coverage Reduction	New Missed Lines	%
/src/coverlet.core/CoverageTracker.cs	1	66.67%

Totals
Change from base Build #20:	-0.2%
Covered Lines:	302
Relevant Lines:	364

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💛 - Coveralls

[tonerdo]

0 coverage will cause errors because no HitsFile

This is a very compelling point. Once the tests pass, I'll merge this in and accept the other guy's PR and do some personal verifications. I figure as long as the final summary explicitly states 0% as opposed to just failing silently, we should be in a good place