Fix null pointer that was seen while investigating issue #161.  Fix by checking that map contains key first.
Fix null pointer that was seen while investigating issue #161.  Fix by checking that map contains key first.

https://github.com/couchbase/couchbase-lite-android/issues/161

mariosotil/couchbase-lite-java-core,deleet/couchbase-lite-java-core,couchbase/couchbase-lite-java-core,Spotme/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,netsense-sas/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,4u7/couchbase-lite-java-core,gotmyjobs/couchbase-lite-java-core