quic: fix buffer slicing when handling overlapping stream data

When processing a STREAM frame that overlaps with already received data,
our slicing calculation is wrong. As a result, upon receiving duplicate
data, such as when packets are retransmitted, we end up writing the same
data multiple times rather than deduplicating them. This seems to have
resulted in spurious FINAL_SIZE_ERROR in our tests.

For golang/go#78737

Change-Id: I0c6d34d7f7f376310dad43983cbdf32b6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/776080
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>