ZSTD_compressSequences_internal assert op <= oend (#1667)
ZSTD_compressSequences_internal assert op <= oend (#1667)

When we wrote one byte beyond the end of the buffer for RLE
blocks back in 1.3.7, we would then have `op > oend`. That is
a problem when we use `oend - op` for the size of the destination
buffer, and allows further writes beyond the end of the buffer for
the rest of the function. Lets assert that it doesn't happen.
Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd