Export of internal Abseil changes.

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2d7c9650fe2b1bf76085a993285e49db4d24aa4f by Derek Mauro <dmauro@google.com>:

Fix a build error in hash_test.cc under MSVC 2017

PiperOrigin-RevId: 249666534

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66887d4ed411663f688fc9852701d648fe7cefe6 by Derek Mauro <dmauro@google.com>:

Merge GitHub #316 - Avoid undefined behavior when passing nullptr to
memcpy with size 0.

Fixes #315

PiperOrigin-RevId: 249570899
GitOrigin-RevId: 2d7c9650fe2b1bf76085a993285e49db4d24aa4f
Change-Id: I0fae0c581f3c0df9e6f70c69661973d020155738