pw_transfer: Fix undefined behavior in memcpy
pw_transfer: Fix undefined behavior in memcpy

Passing nullptr to memcpy is undefined behavior, even when copying 0
bytes. Switch to std::copy to avoid this.

Fixes: 543
Change-Id: I31139435e8d3860003874ad6c21211d476af6577
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/69060
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed