[ast] allow _iRetCount == 0 in gateways

[ast] allow _iRetCount == 0 in gateways

http://bugzilla.scilab.org/show_bug.cgi?id=2036

Just an example of what can be done after this patch

* at the macro level (see plot.sci in the patch):

--> plot(1:10,sin(1:10),1:10,cos(1:10))

--> h=plot(1:10,sin(1:10),1:10,cos(1:10))
 h  =

2 by 1 matrix of handles:
=========================
Polyline
Polyline

--> plot(1:10,sin(1:10),1:10,cos(1:10)).foreground
 ans  =

   33.
   2.

zero value of argn(1) is overriden only in relevant cases: e.g. recursive
extraction or use of expression as an argument in another callExp.

* at the gateway level (here old Scilab C API) :

--> plot3d(rand(10,10))

--> h=plot3d(rand(10,10))
 h  =

Handle of type "Plot3d" with properties:
========================================
parent: Axes
children: []
visible = "on"
surface_mode = "on"
foreground = -1
thickness = 1
mark_mode = "off"
mark_style = 0
mark_size_unit = "tabulated"
mark_size = 0
mark_foreground = -1
mark_background = -2
data.x = [1,2,3,4,5,6,7,8,9,10]
data.y = [1,2,3,4,5,6,7,8,9,10]
data.z matrix 10x10
color_mode = 2
color_flag = 0
hiddencolor = 4
clip_state = "clipgrf"
clip_box = []
ambient_color = [0,0,0]
diffuse_color = [1,1,1]
specular_color = [1,1,1]
use_color_material = "on"
material_shininess = 2
user_data = []
tag = ""

Change-Id: I60f075f8e348fea51ca6ddc2c7985324de5d63f5