src/xpath.c: coverity fix - resource double free

Author: jsc-se

src/xpath.c

@@ -3666,7 +3666,9 @@ warn_equality_value(const struct lyxp_expr *exp, struct lyxp_set *set, uint32_t
                 type->plugin->free(set->ctx, &storage);
             }
         }
-        free(value);
+        if (value) {
+            free(value);
+        }
     }
 }