Appstream crashes with a short search

[aleixpol]

$ appstreamcli search ka
**
ERROR:/home/apol/devel/frameworks/appstream/src/as-data-pool.c:1024:as_data_pool_search: assertion failed: (terms)
Aborted (core dumped)

as_utils_search_token_valid returns null if the string is too short. I'd say it's ok to restrict broad searches, but I'd rather limit it from the frontend.

[ximion]

Actually, we should IMHO just return everything for short searches, like we did previously (it's a nice way of telling the user "your search is too broad").
Oh yeah, and we should never crash.

[ximion]

And fixed, thanks for reporting! (this helps to improve the unit tests for searching ^^) :)