vbox: fix compilation error

clang complains about possibly uninitialized variable:

vbox/vbox_snapshot_conf.c:1355:9: error: variable 'ret' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
    if (!(xPathContext = xmlXPathNewContext(xml))) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So init 'ret' with NULL.

src/vbox/vbox_snapshot_conf.c

@@ -1336,7 +1336,7 @@ virVBoxSnapshotConfGetRODisksPathsFromLibvirtXML(const char *filePath,
 {
     int result = -1;
     size_t i = 0;
-    char **ret;
+    char **ret = NULL;
     xmlDocPtr xml = NULL;
     xmlXPathContextPtr xPathContext = NULL;
     xmlNodePtr *nodes = NULL;