The referenced Minidump file from breakpad should show the backtrace of where the crash occurred. You can convert this into a core file using minidump-2-core - e.g.
/opt/couchbase/bin/minidump-2-core XXX.dmp > mcd_crash.core
If you then install the symbol files for your version (likely https://packages.couchbase.com.s3.amazonaws.com/releases/4.5.0/couchbase-server-community-dbg_4.5.0-ubuntu14.04_amd64.deb) and then run GDB you should be able to get a symbolised backtrace of the crash:
gdb -c mdc_crash.core /opt/couchbase/bin/memcached
thread apply all backtrace full
If you add that to this thread (along with the previous 100 or so lines from the memcached.log leading up to the crash) we can see if it’s a known issue or something new.