Fix use of expr_alt in fuzzer that was causing the build to fail

Author: marijnh

src/fuzzer/fuzzer.rs

@@ -69,7 +69,7 @@ pure fn safe_to_use_expr(e: ast::expr, tm: test_mode) -> bool {
           ast::expr_if(_, _, _) { false }
           ast::expr_if_check(_, _, _) { false }
           ast::expr_block(_) { false }
-          ast::expr_alt(_, _) { false }
+          ast::expr_alt(_, _, _) { false }
           ast::expr_for(_, _, _) { false }
           ast::expr_while(_, _) { false }