core: fix panic when AllocatedResources is nil

Potential fix for #6540

scheduler/util.go

@@ -838,13 +838,16 @@ func genericAllocUpdateFn(ctx Context, stack Stack, evalID string) allocUpdateTy
 			Tasks: option.TaskResources,
 			Shared: structs.AllocatedSharedResources{
 				DiskMB: int64(newTG.EphemeralDisk.SizeMB),
-				// Since this is an inplace update, we should copy network
-				// information from the original alloc. This is similar to
-				// how we copy network info for task level networks above.
-				Networks: existing.AllocatedResources.Shared.Networks,
 			},
 		}
 
+		// Since this is an inplace update, we should copy network
+		// information from the original alloc. This is similar to how
+		// we copy network info for task level networks above.
+		if existing.AllocatedResources != nil {
+			newAlloc.AllocatedResources.Shared.Networks = existing.AllocatedResources.Shared.Networks
+		}
+
 		// Use metrics from existing alloc for in place upgrade
 		// This is because if the inplace upgrade succeeded, any scoring metadata from
 		// when it first went through the scheduler should still be preserved. Using scoring