Use a try_from instead of a cast

This is part of #101.

Author: pierwill

src/protocol/frame.rs

@@ -42,11 +42,10 @@ where
         let len = Int32::read(&mut Cursor::new(len_buf))
             .expect("Reading Int32 from in-mem buffer should always work");
 
-        // convert to usize
-        if len.0 < 0 {
-            return Err(ReadError::NegativeMessageSize { size: len.0 });
-        }
-        let len = len.0 as usize;
+        let len = match usize::try_from(len.0) {
+            Ok(n) => n,
+            Err(_) => return Err(ReadError::NegativeMessageSize { size: len.0 }),
+        };
 
         // check max message size to not blow up memory
         if len > max_message_size {