Unchecked cb_malloc and cb_calloc in couchstore

As part of MB-46221, much analysis and review of couchstore was done looking for a crash, this lead to the identification of two unchecked memory allocation calls.

These two unchecked calls are present in 6.6.3 and should be backported.

• http://review.couchbase.org/c/couchstore/+/153669
• http://review.couchbase.org/c/couchstore/+/153678