add some missing list get/set bounds checks

Author: dwrensha

capnp/src/primitive_list.rs

@@ -157,6 +157,7 @@ where
     }
 
     pub fn set(&mut self, index: u32, value: T) {
+        assert!(index < self.len());
         PrimitiveElement::set(&self.builder, index, value);
     }
 

capnp/src/struct_list.rs

@@ -185,6 +185,7 @@ where
     where
         T::Reader<'b>: crate::traits::IntoInternalStructReader<'b>,
     {
+        assert!(index < self.len());
         use crate::traits::IntoInternalStructReader;
         self.builder
             .reborrow()