avoid needless call to memcpy with an empty entries_ array

[jmalkin]

Based on #300 , we should avoid calling memcpy() when entries_ is empty to keep -fsanitize=undefiend happy. The sanitizer (reasonably) detected a copy of 0 bytes from nullptr. And while that should be completely harmless in this case, it's cleaner to avoid entirely.

[coveralls]

Pull Request Test Coverage Report for Build 2951678780

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.347%

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Totals
Change from base Build 2885525020:	0.0%
Covered Lines:	2196
Relevant Lines:	2378

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💛 - Coveralls

[AlexanderSaydakov]

fixes #300