virsh: Fix core for cmdSecretGetValue

https://bugzilla.redhat.com/show_bug.cgi?id=1366611

When commit id 'cb2e3e50' reworked the cmdSecretGetValue call to use
VIR_DISPOSE_STRING for base64, it neglected to initialize the base64
value to NULL since the cleanup: label could be reached prior to the
base64 value being set or not.  This resulted in a core dump, adding
the initialization will avoid the issue.

tools/virsh-secret.c

@@ -253,7 +253,7 @@ static bool
 cmdSecretGetValue(vshControl *ctl, const vshCmd *cmd)
 {
     virSecretPtr secret;
-    char *base64;
+    char *base64 = NULL;
     unsigned char *value;
     size_t value_size;
     bool ret = false;