`nom::Err::Incomplete` returns total bytes instead of additional bytes

[gerard-ryan-immersaview]

When the ng_block_parse returns the nom::Err::Incomplete error, it returns the total number of bytes that are required for the header, at least in some cases. This is different from the nom::Err::Incomplete documented expectations which says it contains only the additional bytes required.

With the linked example this would mean

-            return Err(nom::Err::Incomplete(nom::Needed::new(P::HDR_SZ)));
+            return Err(nom::Err::Incomplete(nom::Needed::new(P::HDR_SZ - i.len())));

But it appears there is more than just that case.