Clear the dial_string variables

Clearing dial_string prevents poisoning the value for extensions in the ring group destinations that exist but are not registered.

app/switch/resources/scripts/app/ring_groups/index.lua

@@ -915,6 +915,9 @@
 							end
 						end
 
+					--clear the dial_string variables
+						dial_string = nil;
+
 					--increment the value of x
 						x = x + 1;
 				end