fix incorrect use of loop variable (#16872)

This fixes a couple of references to loop variables in parallel tests
and deferred functions. When running a parallel test (calling
`t.Parallel()`) combined with the table-driven pattern, it's necessary
to copy the test case loop variable, otherwise only the last test case
is exercised. This is documented in the `testing` package:

https://pkg.go.dev/testing#hdr-Subtests_and_Sub_benchmarks

`defer` statements that invoke a closure should also not reference a
loop variable directly as the referenced value will change in each
iteration of the loop.

Issues were automatically found with the `loopvarcapture` linter.

command/server.go

@@ -1337,16 +1337,20 @@ func (c *ServerCommand) Run(args []string) int {
 		return 1
 	}
 
-	if seals != nil {
-		for _, seal := range seals {
-			// Ensure that the seal finalizer is called, even if using verify-only
-			defer func(seal *vault.Seal) {
-				err = (*seal).Finalize(context.Background())
-				if err != nil {
-					c.UI.Error(fmt.Sprintf("Error finalizing seals: %v", err))
-				}
-			}(&seal)
+	for _, seal := range seals {
+		// There is always one nil seal. We need to skip it so we don't start an empty Finalize-Seal-Shamir
+		// section.
+		if seal == nil {
+			continue
 		}
+		seal := seal // capture range variable
+		// Ensure that the seal finalizer is called, even if using verify-only
+		defer func(seal *vault.Seal) {
+			err = (*seal).Finalize(context.Background())
+			if err != nil {
+				c.UI.Error(fmt.Sprintf("Error finalizing seals: %v", err))
+			}
+		}(&seal)
 	}
 
 	if barrierSeal == nil {