dynamic-string: Fix undefined behavior due to offsetting null pointer.

When compiled with clang and '-fsanitize=undefined' set, running
'ovsdb-client --timestamp monitor Open_vSwitch' in a sandbox triggers
the following undefined behavior (flagged by UBSan):

  lib/dynamic-string.c:207:38: runtime error: applying zero offset to null pointer
      #0 0x4ebc18 in ds_put_strftime_msec lib/dynamic-string.c:207:38
      #1 0x4ebd04 in xastrftime_msec lib/dynamic-string.c:225:5
      #2 0x552e6a in table_format_timestamp__ lib/table.c:226:12
      #3 0x552852 in table_print_timestamp__ lib/table.c:233:27
      #4 0x5506f3 in table_print_table__ lib/table.c:254:5
      #5 0x550633 in table_format lib/table.c:601:9
      #6 0x5524f3 in table_print lib/table.c:633:5
      #7 0x44dc5e in monitor_print_table ovsdb/ovsdb-client.c:1019:5
      #8 0x44c650 in monitor_print ovsdb/ovsdb-client.c:1040:13
      #9 0x44ac56 in do_monitor__ ovsdb/ovsdb-client.c:1500:21
      #10 0x44636e in do_monitor ovsdb/ovsdb-client.c:1575:5
      #11 0x442c41 in main ovsdb/ovsdb-client.c:283:5

Reported-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/dynamic-string.c

@@ -202,10 +202,11 @@ ds_put_strftime_msec(struct ds *ds, const char *template, long long int when,
         localtime_msec(when, &tm);
     }
 
+    ds_reserve(ds, 64);
     for (;;) {
-        size_t avail = ds->string ? ds->allocated - ds->length + 1 : 0;
-        size_t used = strftime_msec(&ds->string[ds->length], avail, template,
-                                    &tm);
+        size_t avail = ds->allocated - ds->length + 1;
+        char *dest = &ds->string[ds->length];
+        size_t used = strftime_msec(dest, avail, template, &tm);
         if (used) {
             ds->length += used;
             return;