Answer
These observations do not violate the law of conservation of mass.
Work Step by Step
When an iron object rusts, it reacts with oxygen in the presence of water (liquid or gas). So according to the law of conservation of mass, the mass of the oxygen has to be added at the products side. That explains why the mass of the iron and the rust after the reaction is more than the mass of the iron before the reaction.
When a match burns it reacts with oxygen to form $CO_{2} (g)$ and $H_{2}O (g)$. According to the law of conservation of mass the masses of both the gaseous reactants has to be added at the reactants side. That accounts for the fact that the mass of the solid match decreases when it burns.
In other words, both examples do not violate the law of conservation of mass, since the mass at the products side and the mass at the reactants side are equal.
