Fixed the following.

runtime error: Cannot read null.netnum
proc name: mergeConnectedNetworksOnTurf (/obj/cable/proc/mergeConnectedNetworksOnTurf)
  source file: cable.dm,497
  usr: Sergio Mitchell (/mob/living/carbon/human)
  src: the  cable (/obj/cable)
  call stack:

git-svn-id: http://tgstation13.googlecode.com/svn/trunk@1926 316c924e-a436-60f5-8080-3fe189b3f50e

code/modules/power/cable.dm

@@ -455,6 +455,10 @@
 	TB = get_step(src, direction)
 
 	for(var/obj/cable/TC in TB)
+
+		if(!TC)
+			continue
+
 		if(src == TC)
 			continue
 
@@ -480,6 +484,10 @@
 
 	for(var/obj/cable/C in loc)
 
+
+		if(!C)
+			continue
+
 		if(C == src)
 			continue
 		if(netnum == 0)
@@ -495,20 +503,28 @@
 
 	for(var/obj/machinery/power/M in loc)
 
-		if(M.netnum < 0)
+		if(!M)
 			continue
-		if(M.netnum == 0)
+
+		if(!M.netnum)
 			var/datum/powernet/PN = powernets[netnum]
 			PN.nodes += M
 			M.netnum = netnum
 			M.powernet = powernets[M.netnum]
 
+		if(M.netnum < 0)
+			continue
+
 		var/datum/powernet/PN = powernets[netnum]
 		var/datum/powernet/TPN = powernets[M.netnum]
 
 		PN.merge_powernets(TPN)
 
 	for(var/obj/machinery/power/apc/N in loc)
+
+		if(!N)
+			continue
+
 		var/obj/machinery/power/M
 		M = N.terminal