adb: fix zero-initialization in Block.

Iccfe3bd4fe45a0319bd9f23b8cbff4c7070c9f4d changed Block from using
malloc to std::make_unique, which does the equivalent of
`new char[size]()`, which value initializes the array members to 0.
Switch to `reset(new char[size])` to avoid this costly initialization.

Test: adb_benchmark
Change-Id: I09aacb949a7bd4a946ce35a8ee65d1f451577b72