Raise error in AST constructor when `@tree` is `nil`

[gazayas]

No error is raised when the syntax in the source code passed to Ripper.sexp is incorrect:

Ripper.sexp("1 +")
#=> nil

We should actually be getting this syntax error:

syntax error, unexpected end-of-input

[gazayas]

It can be confusing when trying to search for things with Masamune, so it would be better to just raise an error from the get go

"hello""

Raises this error:

unterminated string meets end of file
"hello""

Masamune currently creates an object without a problem and performs methods like strings without being aware that the original code is faulty.

irb(main):001:0> require "masamune"
=> true
irb(main):002:0> Masamune::AbstractSyntaxTree.new("\"hello\"\"").strings
=> []
irb(main):003:0> Masamune::AbstractSyntaxTree.new("\"hello\"\"").tree
=> nil