Fix ICE in ptr_arg

clippy_lints/src/ptr.rs

@@ -649,7 +649,7 @@ fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
                     },
                     _ => {
                         skip_count += 1;
-                        results[arg.idx].skip = true;
+                        results[i].skip = true;
                         None
                     },
                 }

tests/ui/crashes/ice-8386.rs

@@ -0,0 +1,3 @@
+fn f(x: u32, mut arg: &String) {}
+
+fn main() {}