Address memory leak on error

*** CID 1440370:  Resource leaks  (RESOURCE_LEAK)
/lib/dns/dst_api.c: 805 in dst_key_fromgssapi()
799     	}
800     
801     	key->keydata.gssctx = gssctx;
802     	*keyp = key;
803     	result = ISC_R_SUCCESS;
804     out:
   CID 1440370:  Resource leaks  (RESOURCE_LEAK)
   Variable "key" going out of scope leaks the storage it points to.
805     	return result;
806     }
807     
808     isc_result_t
809     dst_key_buildinternal(const dns_name_t *name, unsigned int alg,
810     		      unsigned int bits, unsigned int flags,