prevent ice when threshold is 0 and enum has no variants

clippy_lints/src/enum_variants.rs

@@ -167,7 +167,10 @@ fn check_variant(cx: &LateContext<'_>, threshold: u64, def: &EnumDef<'_>, item_n
         return;
     }
 
-    let first = &def.variants[0].ident.name.as_str();
+    let first = match &def.variants.get(0) {
+        Some(variant) => variant.ident.name.as_str(),
+        None => return,
+    };
     let mut pre = camel_case_split(first);
     let mut post = pre.clone();
     post.reverse();