Address GCC 8.3 -O3 compilation warning

Compiling with -O3 triggers the following warning with GCC 8.3:

    driver.c: In function ‘dlz_findzonedb’:
    driver.c:191:29: warning: ‘%u’ directive output may be truncated writing between 1 and 5 bytes into a region of size between 0 and 99 [-Wformat-truncation=]
      snprintf(buffer, size, "%s#%u", addr_buf, port);
                                 ^~
    driver.c:191:25: note: directive argument in the range [0, 65535]
      snprintf(buffer, size, "%s#%u", addr_buf, port);
                             ^~~~~~~
    driver.c:191:2: note: ‘snprintf’ output between 3 and 106 bytes into a destination of size 100
      snprintf(buffer, size, "%s#%u", addr_buf, port);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Increase the size of the relevant array to prevent this warning from
being triggered.

bin/tests/system/dlzexternal/driver.c

@@ -166,7 +166,7 @@ del_name(struct dlz_example_data *state, struct record *list,
 
 static isc_result_t
 fmt_address(isc_sockaddr_t *addr, char *buffer, size_t size) {
-	char addr_buf[100];
+	char addr_buf[INET6_ADDRSTRLEN];
 	const char *ret;
 	uint16_t port = 0;