conf: Fix possible NULL dereference in virStorageVolTargetDefFormat

Commit dae1568c converted the perms
member of the virStorageVolTarget struct into a pointer to make it
optional. But virStorageVolTargetDefFormat did not check perms for
NULL before dereferencing it.

src/conf/storage_conf.c

@@ -1423,6 +1423,7 @@ virStorageVolTargetDefFormat(virStorageVolOptionsPtr options,
         virBufferAsprintf(buf, "<format type='%s'/>\n", format);
     }
 
+    if (def->perms) {
         virBufferAddLit(buf, "<permissions>\n");
         virBufferAdjustIndent(buf, 2);
 
@@ -1439,6 +1440,7 @@ virStorageVolTargetDefFormat(virStorageVolOptionsPtr options,
 
         virBufferAdjustIndent(buf, -2);
         virBufferAddLit(buf, "</permissions>\n");
+    }
 
     if (def->timestamps) {
         virBufferAddLit(buf, "<timestamps>\n");