conf: prevent NULL pointer access in virSecurityLabelDefsParseXML

When checking for seclabels without security models, def->nseclabels is
already set to n. In the case of an error def->seclabels is freed but
nseclabels is left untouched. This leads to a segmentation fault when
def is freed in virDomainDefParseXML.

src/conf/domain_conf.c

@@ -3179,6 +3179,7 @@ error:
         virSecurityLabelDefFree(def->seclabels[i - 1]);
     }
     VIR_FREE(def->seclabels);
+    def->nseclabels = 0;
     VIR_FREE(list);
     return -1;
 }