MDEV-13459 Warnings, when compiling with gcc-7.x

mostly caused by -Wimplicit-fallthrough

client/mysqldump.c

@@ -2012,6 +2012,7 @@ static void print_xml_comment(FILE *xml_file, size_t len,
     case '-':
       if (*(comment_string + 1) == '-')         /* Only one hyphen allowed. */
         break;
+      /* fall through */
     default:
       fputc(*comment_string, xml_file);
       break;