replace use of ptsname_r by ptsname

Index: src/readline_curses.cc
--- src/readline_curses.cc.orig
+++ src/readline_curses.cc
@@ -843,10 +843,8 @@ readline_curses::start()
     this->rc_pty[RCF_MASTER] = openpt_res.unwrap();
     log_perror(grantpt(this->rc_pty[RCF_MASTER]));
     log_perror(unlockpt(this->rc_pty[RCF_MASTER]));
-    char slave_path_str[PATH_MAX];
-    if (ptsname_r(
-            this->rc_pty[RCF_MASTER], slave_path_str, sizeof(slave_path_str))
-        == -1)
+    char *slave_path_str;
+    if ((slave_path_str = ptsname(this->rc_pty[RCF_MASTER])) == nullptr)
     {
         perror("ptsname_r");
         throw error(errno);
