AC_Fence: fixed pre-arm check for polygon fences

for polygon fences we need to check if the vehicle has a position and
is inside the polygon

libraries/AC_Fence/AC_Fence.cpp

@@ -388,6 +388,17 @@ bool AC_Fence::pre_arm_check_polygon(char *failure_msg, const uint8_t failure_ms
         return false;
     }
 
+    Location loc;
+    if (!AP::ahrs().get_location(loc)) {
+        hal.util->snprintf(failure_msg, failure_msg_len, "Fence requires position");
+        return false;
+    }
+
+    if (_poly_loader.breached(loc)) {
+        hal.util->snprintf(failure_msg, failure_msg_len, "Vehicle outside fence");
+        return false;
+    }
+
     return true;
 }