Fix UB in small_vector
Fix UB in small_vector

Summary:
`moveObjectsRightAndCreate()` did pointer arithmetic outside of the object,
because it had a one before the beginning pointer. Instead, decrement the
pointers before use.

Reviewed By: ot

Differential Revision: D14887926

fbshipit-source-id: 198151d1eaef17e86cda800367480e0a8f7b2f49

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly