QmlJS: Fix member completion at the end of a nested object definition.

The parser's error recovery generates three nested object definitions
for code like
Item { Item { font } }
where the innermost definition corresponds to 'font }' and has an empty
lbrace token.

If we create a Range for this recovered definition, code completion will
fail for 'font.'.

Task-number: QTCREATORBUG-2767
Reviewed-by: Roberto Raggi