MDEV-20516: Assertion `!lex->proc_list.first && !lex->result && !lex->param_list.elements' failed in mysql_create_view

Execution of the CREATE VIEW statement sent via binary protocol
where the flags of the COM_STMT_EXECUTE request a cursor to be opened
before running the statement results in an assert failure.

This assert fails since the data member thd->lex->result has not null
value pointing to an instance of the class Select_materialize.
The data member thd->lex->result is assigned a pointer to the class
Select_materialize in the function mysql_open_cursor() that invoked
in case the packet COM_STMT_EXECUTE requests a cursor to be opened.

After thd->lex->result is assigned a pointer to an instance of the
class Select_materialize the function mysql_create_view() is called
(indirectly via the function mysql_execute_statement()) and the assert
fails.

The assert
  DBUG_ASSERT(!lex->proc_list.first && !lex->result &&
              !lex->param_list.elements);

was added by the commit 591c06d4.
Unfortunately , the condition
  !lex->result
was specified incorrect. It was supposed that the thd->lex->result
is set only by parser on handling the clauses SELECT ... INTO
but indeed it is also set inside mysql_open_cursor() and
that fact was missed by the assert's condition.

So, the fix for this issue is to just remove the condition
  !lex->result
from the failing assert.