dcerpc: move drep alloc on stack

DCERPC's drep field is always 4 bytes for connection oriented PDU types.
Make it a byte array of exactly 4 bytes to store the entire header on
stack.

rust/src/dcerpc/dcerpc.rs

@@ -278,13 +278,13 @@ pub struct Uuid {
     pub node: Vec<u8>,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub struct DCERPCHdr {
     pub rpc_vers: u8,
     pub rpc_vers_minor: u8,
     pub hdrtype: u8,
     pub pfc_flags: u8,
-    pub packed_drep: Vec<u8>,
+    pub packed_drep: [u8; 4],
     pub frag_length: u16,
     pub auth_length: u16,
     pub call_id: u32,

rust/src/dcerpc/parser.rs

@@ -233,7 +233,12 @@ pub(super) fn parse_dcerpc_header(i: &[u8]) -> IResult<&[u8], DCERPCHdr> {
         rpc_vers_minor,
         hdrtype,
         pfc_flags,
-        packed_drep: packed_drep.to_vec(),
+        packed_drep: [
+            packed_drep[0],
+            packed_drep[1],
+            packed_drep[2],
+            packed_drep[3],
+        ],
         frag_length,
         auth_length,
         call_id,