Remove panic in Parser::next

[Mrmaxmeier]

Hey,
I noticed this panic while passing invalid GraphQL queries to juniper.
I've moved it to the ParseError::UnexpectedEndOfFile error instead of panicking.

[mhallin]

Neat! This was a case I thought was unreachable :) Do you have an example of a query that panics the current implementation?

[Mrmaxmeier]

The queries I tried were "mutation" and "query".
I've just created a fuzz target for cargo-fuzz and found some other interesting ones:

• ""
• "\t"
• "{..."
• "))))"
• "$ \""

[mhallin]

Nice finds, thanks a lot.