Fix crash caused by unregistered enum types

Fix crash caused by unregistered enum types

If the enum type isn't registered with Q_ENUMS, the metatype lookup
fails, which results in a crash in certain circumstances.
This commit ensures that when assigning an undefined value to an
unregistered-enum-type property, no crash occurs.

Change-Id: I0b539b591c9c9d6262c748300e4f4b6813d4f9a6
Reviewed-by: Martin Jones <martin.jones@nokia.com>