Check for zero-length binary streams

src/preserves.nim

@@ -545,6 +545,7 @@ proc encode*[E](pr: Preserve[E]): seq[byte] =
 
 proc decodePreserves*(s: Stream; E = void): Preserve[E] =
   ## Decode a Preserves value from a binary-encoded stream.
+  if s.atEnd: raise newException(ValueError, "End of Preserves stream")
   proc assertStream(check: bool) =
     if not check:
       raise newException(ValueError, "invalid Preserves stream")