qapi/error: assert QAPISourceInfo is not None

qapi/error: assert QAPISourceInfo is not None

Built-in stuff is not parsed from a source file, and therefore have no
QAPISourceInfo. If such None info was used for reporting an error,
built-in stuff would be broken. Programming error. Instead of reporting
a confusing error with bogus source location then, we better crash.

We currently crash only if self.col was set. Assert that self.info is
not None in order to crash reliably.

We can not yet change the type of the initializer to prove this cannot
happen at static analysis time before the remainder of the code is fully
typed.

Signed-off-by: John Snow <[email protected]>
Message-Id: <20210421192233.3542904[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>