resolve panic from malformed filter argument

src/parse/args.rs

@@ -341,7 +341,9 @@ impl<'a> Parser<'a> {
                                 return Err(("expected \")\".", pos).into());
                             }
                         }
-                        Some(..) => unreachable!(),
+                        Some(Token { pos, .. }) => {
+                            return Err(("expected \")\".", *pos).into());
+                        }
                         None => return Err(("expected \")\".", span).into()),
                     }
                 }

tests/args.rs

@@ -172,6 +172,10 @@ error!(
     unclosed_paren_in_nested_args,
     "a { color: a(b(red); }", "Error: expected \")\"."
 );
+error!(
+    filter_rhs_missing_closing_paren,
+    "a { color: lighten(red=(green); }", "Error: expected \")\"."
+);
 test!(
     space_after_loud_comment,
     "@mixin foo($x) {