memtx: assert that space is not NULL in index_read_view_create

This should suppress the following coverity issues:

https://scan7.scan.coverity.com/reports.htm#v39198/p13437/fileInstanceId=146712118&defectInstanceId=18978766&mergedDefectId=1563095
https://scan7.scan.coverity.com/reports.htm#v39198/p13437/fileInstanceId=146712113&defectInstanceId=18978750&mergedDefectId=1563094

While we are at it, let's use space_by_id instead of space_cache_find
because read view creation is a rare operation affecting all spaces so
caching the last space by id doesn't make any sense.

NO_DOC=code health
NO_TEST=code health
NO_CHANGELOG=code health

(cherry picked from commit f2886dd0)