Fix SEGV when give invalid object to variables.

[ksss]

I found another pattern to SEGV.

RBS::Parser.parse_type("bool", variables: 1)

This patch raises a TypeError and produces the following message

wrong argument type Integer (must be array or nil) (TypeError)

[soutaro]

@ksss Thank you very much. 🙏 I'm curious what if we pass an array of something not symbol.

RBS::Parser.parse_type("bool", variables: [1])

I'm not sure if I implemented something to handle that case...

[soutaro]

Confirmed no SEGV happens when we pass an array of object.

TypeError: wrong argument type Object (expected Symbol)

[soutaro]

Thanks!

[ksss]

It seems that SYM2ID checks if the value is other than a symbol 👍