Fix execute not to mangle the argument when it prompts for it

The argument is UTF-8.  If it's missing, execute() gets it with
getstring(), which mangles non-ASCII characters.  Clients reject
mangled file names for security reasons.

Fix by switching to ugetstring().  Missed in commit 69832255, v4.3.11.

Non-ASCII characters are still mangled in ASCII sessions, but that's
not expected to work.

src/lib/player/player.c

@@ -242,18 +242,17 @@ make_stale_if_command_arg(char *arg)
 int
 execute(void)
 {
-    char buf[1024];
+    char buf[1024];		/* UTF-8 */
     int failed;
-    char *p;
+    char *p;			/* UTF-8 */
     char *redir;		/* UTF-8 */
     char scanspace[1024];
 
     failed = 0;
 
-    if (player->comtail[1])
-	p = player->comtail[1];
-    else
-	p = getstring("File? ", buf);
+    p = player->comtail[1];
+    if (!p)
+	p = ugetstring("File? ", buf);
     if (p == NULL || *p == '\0')
 	return RET_SYN;
     prexec(p);