cli: fix panic on job restart (#17346)

When monitoring the replacement allocation, if the
`Allocations().Info()` request fails, the `alloc` variable is `nil`, so
it should not be read.

.changelog/17346.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+cli: Fix a panic in the `nomad job restart` command when monitoring replacement allocations
+```

command/job_restart.go

@@ -1045,7 +1045,7 @@ func (c *JobRestartCommand) monitorReplacementAlloc(
 
 		alloc, qm, err := c.client.Allocations().Info(currentAllocID, q)
 		if err != nil {
-			errCh <- fmt.Errorf("Failed to retrieve allocation %q: %w", limit(alloc.ID, c.length), err)
+			errCh <- fmt.Errorf("Failed to retrieve allocation %q: %w", limit(currentAllocID, c.length), err)
 			return
 		}