fix(server): Panic when last envelope item is empty

[jan-auer]

Trailing newlines can be omitted in envelopes. When the last envelope item was
empty, the parsing code could yield two invalid results:

• If an explicit length of 0 was specified, then the envelope was rejected
  with UnexpectedEof.
• If implicit length was used, the parser would panic.

This can both be solved by aligning the parse offset with the end of the buffer.

[jan-auer]

This patch qualifies as critical fix as it resolves a production panic, and is approved for merge during code freeze.