LibLine: Handle read events serially

Previously LibLine accepted read callbacks while it was in the process
of reading input, this wasn't an issue as no async code was being
executed up until the Shell autocompletion came along.
Simply defer input processing while processing input to avoid causing
problems.
Fixes #13280.

Userland/Libraries/LibLine/Editor.cpp

@@ -791,6 +791,13 @@ void Editor::handle_interrupt_event()
 
 void Editor::handle_read_event()
 {
+    if (m_prohibit_input_processing) {
+        m_have_unprocessed_read_event = true;
+        return;
+    }
+
+    auto prohibit_scope = prohibit_input();
+
     char keybuf[16];
     ssize_t nread = 0;