Fix divide by zero crash in zone lighter

code/nel/src/3d/zone_lighter.cpp

@@ -1179,6 +1179,8 @@ void CZoneLighter::light (CLandscape &landscape, CZone& output, uint zoneToLight
 	// Set the thread state
 	_LastPatchComputed.resize (_ProcessCount);
 
+	if (patchCount)
+	{
 		// Launch threads
 		uint process;
 		for (process = 0; process < _ProcessCount; process++)
@@ -1210,6 +1212,11 @@ void CZoneLighter::light (CLandscape &landscape, CZone& output, uint zoneToLight
 			// Call the progress callback
 			progress("Lighting patches", (float)_NumberOfPatchComputed / (float)_PatchInfo.size());
 		}
+	}
+	else
+	{
+		nlwarning("Empty zone");
+	}
 
 	// Reset old thread mask
 	currentThread->setCPUMask (threadMask);