MDEV-261 : mysqtest crashes when assigning variable to result of select , like

let x = `SELECT <something>`

The fix is to detect the condition "no active connection",  to report error and die.
Note, that the check for no active connection was already in place for ordinary commands, 
and was missing only for assign-variable command.

client/mysqltest.cc

@@ -739,6 +739,9 @@ void handle_error(struct st_command*,
                   const char *err_sqlstate, DYNAMIC_STRING *ds);
 void handle_no_error(struct st_command*);
 
+static void handle_no_active_connection(struct st_command* command, 
+  struct st_connection *cn, DYNAMIC_STRING *ds);
+
 #ifdef EMBEDDED_LIBRARY
 
 /* workaround for MySQL BUG#57491 */
@@ -2287,6 +2290,19 @@ void var_query_set(VAR *var, const char *query, const char** query_end)
   DBUG_ENTER("var_query_set");
   LINT_INIT(res);
 
+  if (!mysql)
+  {
+    struct st_command command;
+    memset(&command, 0, sizeof(command));
+    command.query= (char*)query;
+    command.first_word_len= (*query_end - query);
+    command.first_argument= command.query + command.first_word_len;
+    command.end= (char*)*query;
+    command.abort_on_error= 1; /* avoid uninitialized variables */
+    handle_no_active_connection(&command, cur_con, &ds_res);
+    DBUG_VOID_RETURN;
+  }
+
   /* Only white space or ) allowed past ending ` */
   while (end > query && *end != '`')
   {
@@ -2392,6 +2408,12 @@ void var_set_query_get_value(struct st_command *command, VAR *var)
   DBUG_ENTER("var_set_query_get_value");
   LINT_INIT(res);
 
+  if (!mysql)
+  {
+    handle_no_active_connection(command, cur_con, &ds_res);
+    DBUG_VOID_RETURN;
+  }
+
   strip_parentheses(command);
   DBUG_PRINT("info", ("query: %s", command->query));
   check_command_args(command, command->first_argument, query_get_value_args,
@@ -2550,6 +2572,7 @@ void eval_expr(VAR *v, const char *p, const char **p_end, bool do_eval)
       command.first_word_len= len;
       command.first_argument= command.query + len;
       command.end= (char*)*p_end;
+      command.abort_on_error= 1; /* avoid uninitialized variables */
       var_set_query_get_value(&command, v);
       DBUG_VOID_RETURN;
     }
@@ -6924,6 +6947,22 @@ int append_warnings(DYNAMIC_STRING *ds, MYSQL* mysql)
 }
 
 
+/*
+  Handle situation where query is sent but there is no active connection 
+  (e.g directly after disconnect).
+
+  We emulate MySQL-compatible behaviour of sending something on a closed
+  connection.
+*/
+static void handle_no_active_connection(struct st_command *command, 
+  struct st_connection *cn, DYNAMIC_STRING *ds)
+{
+  handle_error(command, 2006, "MySQL server has gone away", "000000", ds);
+  cn->pending= FALSE;
+  var_set_errno(2006);
+}
+
+
 /*
   Run query using MySQL C API
 
@@ -6950,11 +6989,7 @@ void run_query_normal(struct st_connection *cn, struct st_command *command,
 
   if (!mysql)
   {
-    /* Emulate old behaviour of sending something on a closed connection */
-    handle_error(command, 2006, "MySQL server has gone away",
-                 "000000", ds);
-    cn->pending= FALSE;
-    var_set_errno(2006);
+    handle_no_active_connection(command, cn, ds);
     DBUG_VOID_RETURN;
   }