Fix interpreter-exec crash

PR mi/10347 points out that using interpreter-exec inside of a
"define" command will crash gdb.  The bug here is that
gdb_setup_readline doesn't check for the case where instream==nullptr.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=10347

gdb/event-top.c

@@ -1332,8 +1332,10 @@ gdb_setup_readline (int editing)
 
   /* If the input stream is connected to a terminal, turn on editing.
      However, that is only allowed on the main UI, as we can only have
-     one instance of readline.  */
-  if (ISATTY (ui->instream) && editing && ui == main_ui)
+     one instance of readline.  Also, INSTREAM might be nullptr when
+     executing a user-defined command.  */
+  if (ui->instream != nullptr && ISATTY (ui->instream)
+      && editing && ui == main_ui)
     {
       /* Tell gdb that we will be using the readline library.  This
 	 could be overwritten by a command in .gdbinit like 'set

gdb/testsuite/gdb.base/interp.exp

@@ -79,6 +79,17 @@ gdb_test_multiple "interpreter-exec mi3 \"-break-insert main\"" "" {
     }
 }
 
+set test "define hello command"
+set commands "interpreter-exec mi3 \"-data-evaluate-expression 23\""
+gdb_test_multiple "define hello" "$test" {
+    -re "Type commands for definition of \"hello\".\r\nEnd with a line saying just \"end\".\r\n>$" {
+	pass "$test"
+    }
+}
+gdb_test "$commands\nend" "" "finish defining hello command"
+
+gdb_test "hello" [string_to_regexp "^done,value=\"23\""]
+
 if ![runto_main] then {
   return -1
 }