gdb: fix possible use-after-free when executing commands

In principle, `execute_command()` does following:

   struct cmd_list_element *c;
   c = lookup_cmd ( ... );
   ...
   /* If this command has been pre-hooked, run the hook first.  */
   execute_cmd_pre_hook (c);
   ...
   /* ...execute the command `c` ...*/
   ...
   execute_cmd_post_hook (c);

This may lead into use-after-free error.  Imagine the command
being executed is a user-defined Python command that redefines
itself.  In that case, struct `cmd_list_element` pointed to by
`c` is deallocated during its execution so it is no longer valid
when post hook is executed.

To fix this case, this commit looks up the command once again
after it is executed to get pointer to (possibly newly allocated)
`cmd_list_element`.