Turn on buffering when producing hex dumps.

- To produce hex dumps, we need to enable mapping, but also turn
    on buffering.  This makes sure that the map contains the whole
    packet content, even if it has not been parsed (such as when
    encountering unknown or junk pseudo packets).

  - Fixes a crash when hex dumping pseudo packets created by the
    packet parser for junk data in the packet stream.

  - Fixes #201.

src/commands/toolbox/packet/dump.rs

@@ -44,8 +44,18 @@ pub fn dump<W>(config: &crate::Config,
     where W: Into<Option<usize>>
 {
     let mut ppr
-        = self::openpgp::parse::PacketParserBuilder::from_reader(input)?
-        .map(hex).build()?;
+        = self::openpgp::parse::PacketParserBuilder::from_reader(input)?;
+
+    // To produce hex dumps, we need to enable mapping, but also turn
+    // on buffering.  This makes sure that the map contains the whole
+    // packet content, even if it has not been parsed (such as when
+    // encountering unknown or junk pseudo packets).
+    if hex {
+        ppr = ppr.map(true).buffer_unread_content();
+    }
+
+    let mut ppr = ppr.build()?;
+
     let mut message_encrypted = false;
     let width = width.into().unwrap_or(80);
     let mut dumper = PacketDumper::new(width, mpis);