parser.act: Avoid crash in parser from '(*:'.

See #386 on katef/libfsm.

This is a workaround for a bug in the parser -- once the fuzzer
finds it, it tends to get in the way of finding deeper issues.

src/libre/parser.act

@@ -1095,6 +1095,17 @@
 		err->e = RE_ESUCCESS;
 
 		ADVANCE_LEXER;
+
+#define BUILD_FOR_FUZZER 0
+#if BUILD_FOR_FUZZER
+		/* these errors currently are not handled properly */
+		if (act_state->lex_tok == TOK_ERROR) {
+			fprintf(stderr, "syntax error\n");
+			lx->free(lx->buf_opaque);
+			goto error;
+		}
+#endif
+
 		DIALECT_ENTRY(&flags, lex_state, act_state, err, &ast->expr);
 
 		lx->free(lx->buf_opaque);