pw_stream: Don't memmove with zero bytes
pw_stream: Don't memmove with zero bytes

Bug: 527
Change-Id: Iab4f1789ca64273a8c20c5f9fba1b90050b1a3ac
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/68586
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Commit-Queue: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

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