add is_sized method on Abi and Layout, and use it

src/type_.rs

diff --git a/src/type_.rs b/src/type_.rs
index 58d4f29..55e4d0c 100644
--- a/src/type_.rs
+++ b/src/type_.rs
@@ -277,7 +277,7 @@
         offset = target_offset + field.size;
         prev_effective_align = effective_field_align;
     }
-    if !layout.is_unsized() && field_count > 0 {
+    if layout.is_sized() && field_count > 0 {
         if offset > layout.size {
             bug!("layout: {:#?} stride: {:?} offset: {:?}", layout, layout.size, offset);
         }