BUG#36861912 MySQL Shell behavior on Console Exit

When running Shell via SSH (i.e. putty), if a command was entered in the
prompt and then the SSH connection was closed, it was possible that this
command was executed, even though the enter key was not pressed.

The linenoise module (which is used by Shell to read from terminal) was
not treating a situation where stdin is closed as an error. Instead,
it returned buffer holding the current contents of prompt, which was
then executed by Shell.

The linenoise module was adjusted to signal when stdin is closed, in
such case Shell aborts execution.

Change-Id: I494b0db0929f5b0b95d7fe871b95c4f4dc63246b

Author: Paweł Andruszkiewicz

ext/linenoise-ng/src/linenoise.cpp

@@ -2832,6 +2832,9 @@ int InputBuffer::getInputLine(PromptBase &pi) {
         } else if (c == ctrlChar('D')) {
           keyType = 2;
         }
+      } else {
+        // reading was interrupted (i.e. stdin was closed)
+        keyType = 3;
       }
 
 #ifndef _WIN32

src/mysqlsh/cmdline_shell.cc

@@ -916,6 +916,11 @@ char *Command_line_shell::readline(const char *prompt) {
     console->enable_global_pager();
   }
 
+  if (3 == linenoiseKeyType()) {
+    // reading was interrupted
+    return nullptr;
+  }
+
   if (!tmp) {
     switch (linenoiseKeyType()) {
       case 1:  // ^C