[Bug] v.net.visibility doesn't use the additional points coords when the visibility option is present

[agiudiceandrea]

Describe the bug
According to the docs, v.net.visibility should work in three modes:

1. v.net.visibility input=input_layer output=graph -> computes the visibility graph between points/vertices of input_layer
2. v.net.visibility input=input_layer coordinates=x1,y1,x2,y2,... output=graph -> computes the visibility graph between points/vertices of input_layer plus the additional (x1,y1), (x2,y2), ... points
3. v.net.visibility input=input_layer coordinates=x1,y1,x2,y2,... visibility=old_graph output=new_graph -> copies the previously computed visibility graph and then computes only additional visibility graph between points/vertices of input_layer and the additional (x1,y1), (x2,y2), ... extra points

While modes 1 and 2 seems to work well, mode 3 seems not working properly: the new output graph (new_graph) is always identical to the previously computed visibility graph (old_graph) specified with the visibility parameter as it seems that the coordinates of the point specified with the coordinates parameter are not taken into account.

To Reproduce
Steps to reproduce the behavior:

Follow Example 4 in v.net.visibility docs:

v.extract input=zipcodes_wake output=areas_7_11_25 cats=7,11,25
g.region vector=zipcodes_wake
d.mon wx0
d.vect areas_7_11_25
v.net.visibility input=areas_7_11_25 output=graph
v.net.visibility input=areas_7_11_25 visibility=graph output=new_graph \
      coordinates=669547.97,208348.20

Expected behavior
The vector map graph is computed then a new visibility graph is computed from it with the extra point 669547.97,208348.20

System description (please complete the following information):

• Operating System: Windows
• GRASS GIS version 7.8.5

Additional context

It seems to me there is a flaw in the module logic:
in the main function the n int variable is initially set to 0

grass/vector/v.net.visibility/main.c

Line 36 in 8a9a787

int n = 0;

then the variable n is not properly set to the number of additional extra points (specified with the "coordinates" parameter) before the visibility_points function is called:

grass/vector/v.net.visibility/main.c

Line 113 in 8a9a787

visibility_points(points, num_points, lines, num_lines, &out, n);

so the outermost loop in the visibility_points function is never performed:

grass/vector/v.net.visibility/visibility.c

Lines 328 to 329 in 8a9a787

	/* loop through the points to add */
	for (i = 0; i < n; i++) {

Maybe @mmaldacker could give better insights.

[ninsbl]

Should #1336 be backported?

[neteler]

Backport for 7.8.6 done