MDEV-31366 Assertion `thd->start_time' failed in bool LOGGER::slow_log_print(THD*, const char*, size_t, ulonglong)

Fixing a wrong DBUG_ASSERT.

thd->start_time and thd->start_time_sec_part cannot be 0 at the same time.

But thd->start_time can be 0 when thd->start_time_sec_part is not 0,
e.g. after:

SET timestamp=0.99;

mysql-test/main/log_slow.result

@@ -207,4 +207,21 @@ use `a
 b`;
 SET timestamp=1234567890;
 select count(*) from mysql.global_priv where length(priv)>2
+#
+# MDEV-31366 Assertion `thd->start_time' failed in bool LOGGER::slow_log_print(THD*, const char*, size_t, ulonglong)
+#
+TRUNCATE mysql.slow_log;
+SET global log_output='TABLE';
+SET timestamp=0.99;
+SET long_query_time=0.00001;
+SELECT SLEEP(0.1);
+SLEEP(0.1)
+0
+SELECT COUNT(*)>0 AS c1 FROM mysql.slow_log;
+c1
+1
+SET global log_output=default;
+SET timestamp=default;
+SET long_query_time=default;
+TRUNCATE mysql.slow_log;
 # End of 10.5 tests

mysql-test/main/log_slow.test

@@ -221,4 +221,19 @@ let SEARCH_PATTERN= use \`a\n.*2;
 let SEARCH_OUTPUT=matches;
 source include/search_pattern_in_file.inc;
 
+--echo #
+--echo # MDEV-31366 Assertion `thd->start_time' failed in bool LOGGER::slow_log_print(THD*, const char*, size_t, ulonglong)
+--echo #
+
+TRUNCATE mysql.slow_log;
+SET global log_output='TABLE';
+SET timestamp=0.99;
+SET long_query_time=0.00001;
+SELECT SLEEP(0.1);
+SELECT COUNT(*)>0 AS c1 FROM mysql.slow_log;
+SET global log_output=default;
+SET timestamp=default;
+SET long_query_time=default;
+TRUNCATE mysql.slow_log;
+
 --echo # End of 10.5 tests

sql/log.cc

@@ -1330,7 +1330,7 @@ bool LOGGER::slow_log_print(THD *thd, const char *query, size_t query_length,
                     user_host_buff);
 
     DBUG_ASSERT(thd->start_utime);
-    DBUG_ASSERT(thd->start_time);
+    DBUG_ASSERT(thd->start_time || thd->start_time_sec_part);
     query_utime= (current_utime - thd->start_utime);
     lock_utime=  (thd->utime_after_lock - thd->start_utime);
     my_hrtime_t current_time= { hrtime_from_time(thd->start_time) +