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 <joetsai@google.com>