Add missing NULL termination of stropcode_map

Add a NULL terminator to stropcode_map to prevent a crash in
lcb_cntl_string() if it is passed an unknown control string.

Found when trying to set 'boostrap_on' via the -C HTTP option of
cbc-pillowfight.

Change-Id: I32386485ad4d5e05ec239413c0b2dc86954bbad5
Reviewed-on: http://review.couchbase.org/38666
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Dave Rigby <daver@couchbase.com>