Fix some things assuming lighting corners existed (#1642)

code/modules/lighting/lighting_source.dm

@@ -270,6 +270,8 @@
 		source_turf.luminosity = Ceiling(light_range)
 		for(T in view(Ceiling(light_range), source_turf))
 			for (thing in T.get_corners(source_turf))
+				if(!thing)
+					continue
 				C = thing
 				corners[C] = 0
 			turfs += T
@@ -291,6 +293,8 @@
 	LAZYINITLIST(effect_str)
 	if (needs_update == LIGHTING_VIS_UPDATE)
 		for (thing in  corners - effect_str) // New corners
+			if(!thing)
+				continue
 			C = thing
 			LAZYADD(C.affecting, src)
 			if (!C.active)
@@ -300,6 +304,8 @@
 	else
 		L = corners - effect_str
 		for (thing in L) // New corners
+			if(!thing)
+				continue
 			C = thing
 			LAZYADD(C.affecting, src)
 			if (!C.active)
@@ -308,6 +314,8 @@
 			APPLY_CORNER(C)
 
 		for (thing in corners - L) // Existing corners
+			if(!thing)
+				continue
 			C = thing
 			if (!C.active)
 				effect_str[C] = 0
@@ -316,6 +324,8 @@
 
 	L = effect_str - corners
 	for (thing in L) // Old, now gone, corners.
+		if(!thing)
+			continue
 		C = thing
 		REMOVE_CORNER(C)
 		LAZYREMOVE(C.affecting, src)