r.horizon: variable needs to be reset for point mode (#3441)

The bug is that for part of the angles it results in zeros. It comes up when there are nulls at the edge of the raster, test was added.

raster/r.horizon/main.c

@@ -757,6 +757,7 @@ double horizon_height(void)
 
     tanh0 = 0.;
     length = 0;
+    zp = z_orig;
 
     height = searching();
 

raster/r.horizon/testsuite/test_r_horizon.py

@@ -144,6 +144,12 @@ def test_point_mode_multiple_direction(self):
         stdout = module.outputs.stdout
         self.assertMultiLineEqual(first=ref2, second=stdout)
 
+        # include nulls along the edge
+        self.runModule("g.region", raster="elevation", w="w-100")
+        self.assertModule(module)
+        stdout = module.outputs.stdout
+        self.assertMultiLineEqual(first=ref2, second=stdout)
+
     def test_point_mode_multiple_direction_artificial(self):
         """Test mode with 1 point and multiple directions with artificial surface"""
         module = SimpleModule(