util: fix wrong comparison inside virStoragePermsCopy()

VIR_STRDUP returns -1 if the string copy was not successful. So, the
current comparison/logic is throwing an error when VIR_STRDUP() returns
1. Only when source is NULL, it is considering as a success which is
not right.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>

src/util/virstoragefile.c

@@ -1974,7 +1974,7 @@ virStoragePermsCopy(const virStoragePerms *src)
     ret->uid = src->uid;
     ret->gid = src->gid;
 
-    if (VIR_STRDUP(ret->label, src->label))
+    if (VIR_STRDUP(ret->label, src->label) < 0)
         goto error;
 
     return ret;