Fix incorrect cast int16 func name

[ray6080]

This incorrect naming leads to weird behaviours.
An example on tinysnb:

match (p:person) return to_int16(p.age), to_int32(p.age), p.age;

will produce wrong result for to_int16(p.age).
@andyfengHKU or @acquamarin should take a further look.
I would propose to add a function validity check mechanism in catalog when we register any functions, so probably we can avoid this kind of mistakes later.