sequence: use xmalloc where appropriate

Allocations from fiber region and heap should never fail so it's okay to
use xmalloc for them.

This fixes the following coverity report complaining about unchecked
region_alloc result:

https://scan7.scan.coverity.com/reports.htm#v39198/p13437/fileInstanceId=149497639&defectInstanceId=19021351&mergedDefectId=1525560

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring