Quote $cert_ext_key_usage in renew()

'case' is "immune" to standard word splitting and globbing but
code-style now demands that 'case' does not get treated differently.

Signed-off-by: Richard T Bonhomme <tincantech@protonmail.com>

easyrsa3/easyrsa

@@ -1607,7 +1607,7 @@ Renewal not allowed."
 		sed -n "/X509v3 Extended Key Usage:/{n;s/^ *//g;p;}"
 		)
 
-	case $cert_ext_key_usage in
+	case "$cert_ext_key_usage" in
 		"TLS Web Client Authentication")
 			cert_type=client
 			;;