The generated RootBuilder crashed (undefined behavior on std::vector::back())
when a JSON document's root value was a scalar or null while the schema
declared a non-nullable object or array root. The stack starts empty for
object/array roots, but cbStringData, cbNumberData, and cbBool called
stack_.back() without checking for an empty stack.
Add an empty-stack guard to the three scalar callbacks so they reject
instead of crashing. cbNull already handles the empty-stack case.
Regression tests added for:
- non-nullable object root rejecting null, boolean, number, and string roots
- nullable object root rejecting scalar roots
- nullable array root rejecting scalar roots
Closes#16
weaselbot