COPY instructions shouldn't be allowed to cross the source segment/target window boundary

[mverver-google]

In the encoder source code, it is claimed that it's possible for a COPY instruction to begin in the source segment and extend past its end, into the target window.
https://github.com/google/open-vcdiff/blob/master/src/google/vcencoder.h#L97-L102

And the decoder supports such instructions:
https://github.com/google/open-vcdiff/blob/master/src/vcdecoder.cc#L1209-L1216

However, this contradicts the specification in RFC 3284, Section 3:

A target window T, of length t, may be compared against some source
data segment S, of length s. [..]

Assume that S[j] represents the jth byte in S, and T[k] represents
the kth byte in T. Then, for the delta instructions, we treat the
data windows S and T as substrings of a superstring U, formed by
concatenating them like this:

  S[0]S[1]...S[s-1]T[0]T[1]...T[t-1]

[..]
COPY: This instruction has two arguments, a size x and an address
p in the string U. The arguments specify the substring of U
that must be copied. We shall assert that such a substring
must be entirely contained in either S or T.

(Emphasis mine.) This last sentence seems intended to specifically forbid COPY instructions which cross the source segment/target window boundary.

The open-vcdiff implementation is still compliant because the encoder doesn't generate such instructions, but to avoid confusion about the file format, it might be helpful to:

• In the encoder: remove the comment.
• In the decoder: either reject inputs that contain these invalid instructions, or at least add a comment mentioning that such instructions are non-standard.