folly: fbvector: ubsan: avoid memcpy(dest, nullptr, 0)
folly: fbvector: ubsan: avoid memcpy(dest, nullptr, 0)

Reviewed By: meyering

Differential Revision: D3277138

fbshipit-source-id: 7050d18c256ac15c8122d4af59aa31208b1466fd

floxard/folly,brunomorishita/folly,facebook/folly,rklabs/folly,charsyam/folly,mqeizi/folly,rklabs/folly,rklabs/folly,mqeizi/folly,facebook/folly,Orvid/folly,mqeizi/folly,facebook/folly,floxard/folly,Orvid/folly,Orvid/folly,facebook/folly,brunomorishita/folly,charsyam/folly,mqeizi/folly,rklabs/folly,Orvid/folly,charsyam/folly,floxard/folly,floxard/folly,mqeizi/folly,brunomorishita/folly,facebook/folly,floxard/folly,brunomorishita/folly,Orvid/folly,rklabs/folly,brunomorishita/folly,charsyam/folly