Move reset of insert_values to ensure it's done also during error conditions

This fixed a failure of insert_update.test on some platforms


mysys/thr_alarm.c:
  Fixed problem noticed by valgrind
sql/opt_range.cc:
  Simple optimization for common case
sql/sql_base.cc:
  Safety assert
sql/sql_insert.cc:
  Added comment

sql/sql_insert.cc

@@ -516,6 +516,10 @@ abort:
     ON DUPLICATE KEY ...
     we should be able to refer to sum1 in the ON DUPLICATE KEY part
 
+  WARNING
+    You MUST set table->insert_values to 0 after calling this function
+    before releasing the table object.
+
   RETURN VALUE
     0   OK
     -1  error (message is not sent to user)