Indicate and document unreachable code section

Based on Sergei Golubchik's question in
mariadb-corporation#223 (comment)

libmariadb/mariadb_lib.c

@@ -40,6 +40,7 @@
 #include <time.h>
 #include <mariadb_dyncol.h>
 #include <mariadb_rpl.h>
+#include <my_global.h>
 
 #ifndef __has_feature
 # define __has_feature(x) 0
@@ -1811,6 +1812,20 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
   /* Check if server sends an error */
   if (mysql->protocol_version == 0XFF)
   {
+    /* This code appears to be unreachable.
+     *
+     * 1. `if (mysql->protocol_version == 0XFF)` is checking the first
+     *    byte of the packet received by `ma_net_safe_read` above.
+     * 2. But if the first byte of the packet body received by
+     *    `ma_net_safe_read` had been 0xff, then `ma_net_safe_read`
+     *    would have returned `packet_error`.
+     *    (See its source above, in this same file.)
+     * 3. Therefore, we would have taken the above branch
+     *    (`if (ma_net_safe_read(mysql)  == packet_error)`), and jumped
+     *    straight to `goto error`, skipping this section.
+     */
+    MY_ASSERT_UNREACHABLE();
+
     net_get_error(end, pkt_length - 1, net->last_error, sizeof(net->last_error),
       &net->last_errno, net->sqlstate);
     /* fix for bug #26426 */