frame: reject bad opcodes early

Signed-off-by: Alexey Galakhov <agalakhov@snapview.de>

src/protocol/frame/frame.rs

@@ -342,6 +342,14 @@ impl Frame {
             None
         };
 
+        // Disallow bad opcode
+        match opcode {
+            OpCode::Control(Control::Reserved(_)) | OpCode::Data(Data::Reserved(_)) => {
+                return Err(Error::Protocol(format!("Encountered invalid opcode: {}", first & 0x0F).into()))
+            }
+            _ => ()
+        }
+
         // Make sure `length` is not too big (fits into `usize`).
         if length > usize::max_value() as u64 {
             return Err(Error::Capacity(format!("Message length too big: {}", length).into()));
@@ -361,14 +369,6 @@ impl Frame {
             }
         }
 
-        // Disallow bad opcode
-        match opcode {
-            OpCode::Control(Control::Reserved(_)) | OpCode::Data(Data::Reserved(_)) => {
-                return Err(Error::Protocol(format!("Encountered invalid opcode: {}", first & 0x0F).into()))
-            }
-            _ => ()
-        }
-
         let frame = Frame {
             finished: finished,
             rsv1: rsv1,