internal/impl: fix off-by-one error in message initialization
internal/impl: fix off-by-one error in message initialization

This particular off-by-one can never happen in practice. We will crash
if a field has a number one greater than the maximum field number in the
dense array, but if field number N is in the array we will always put
N+1 in it as well.

Fix the off-by-one anyway.

Change-Id: I8c1304f2fc0d7b91036bde3f7ddb7115c21781ca
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/229278
Reviewed-by: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go