Bug#12406055 post-push fix: unused variable 'num_chars' in optimized build.

Also fixed possibly uninitialized use of need_copy_table_res.

client/sql_string.cc

@@ -188,7 +188,8 @@ bool String::set(double num,uint decimals, CHARSET_INFO *cs)
 #else
 #ifdef HAVE_SNPRINTF
   buff[sizeof(buff)-1]=0;			// Safety
-  int num_chars= snprintf(buff, sizeof(buff)-1, "%.*f",(int) decimals, num);
+  IF_DBUG(int num_chars= )
+    snprintf(buff, sizeof(buff)-1, "%.*f",(int) decimals, num);
   DBUG_ASSERT(num_chars > 0);
   DBUG_ASSERT(num_chars < (int) sizeof(buff));
 #else

sql/sql_table.cc

@@ -6985,7 +6985,7 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
     need_copy_table= ALTER_TABLE_DATA_CHANGED;
   else
   {
-    enum_alter_table_change_level need_copy_table_res;
+    enum_alter_table_change_level need_copy_table_res=ALTER_TABLE_METADATA_ONLY;
     /* Check how much the tables differ. */
     if (compare_tables(table, alter_info,
                        create_info, order_num,