MB-11305 remove double-free of conn in allocate_connection()

As identified by clang static analyser; remove the double free of the
connection object. Note that conn_constructor already cleans up after
itself and free()s all allocated data; so it is incorrect to call
conn_destructor() on a failed construction.

Change-Id: Ie0163914ac7295332ff26cca94db3280ed36e27a
Reviewed-on: http://review.couchbase.org/37831
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>