lib/fb: Assert, instead of silently failing, when creating fbs

lib/fb: Assert, instead of silently failing, when creating fbs

We were either returning 0, or a negative value cast to an unsigned int
for errors and the clients of that API weren't exactly checking
anything.

We're in luck, we can take shortcuts in a testing library to just assert
when an expected error occurs.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>