fix(dict): honor legacy dictionary capacity before copying segments

Delay the legacy dictionary segment copy until the effective content size is
known, preventing writes beyond the caller-provided dictionary capacity.

Test Plan:
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo test --manifest-path rust/cli/Cargo.toml
- make -C tests check V=1

rust/src/dict_builder_zdict.rs

@@ -1140,6 +1140,20 @@ unsafe fn train_from_buffer_unsafe_legacy(
     if content_size < ZDICT_CONTENTSIZE_MIN {
         return dictionary_error(ZstdErrorCode::DictionaryCreationFailed);
     }
+    let max = dict_list[0].pos as usize;
+    let mut current_size = 0usize;
+    let mut count = 1usize;
+    while count < max {
+        current_size += dict_list[count].length as usize;
+        if current_size > max_dict_size {
+            current_size -= dict_list[count].length as usize;
+            break;
+        }
+        count += 1;
+    }
+    dict_list[0].pos = count as u32;
+    content_size = current_size;
+
     let output = unsafe { std::slice::from_raw_parts_mut(dict_buffer.cast::<u8>(), max_dict_size) };
     let mut write_at = max_dict_size;
     for item in dict_list.iter().take(dict_list[0].pos as usize).skip(1) {
@@ -1157,19 +1171,6 @@ unsafe fn train_from_buffer_unsafe_legacy(
             .copy_from_slice(&samples_buffer[item.pos as usize..item.pos as usize + length]);
     }
 
-    let max = dict_list[0].pos as usize;
-    let mut current_size = 0usize;
-    let mut count = 1usize;
-    while count < max {
-        current_size += dict_list[count].length as usize;
-        if current_size > max_dict_size {
-            current_size -= dict_list[count].length as usize;
-            break;
-        }
-        count += 1;
-    }
-    dict_list[0].pos = count as u32;
-    content_size = current_size;
     add_entropy_tables_advanced(
         dict_buffer,
         content_size,