patch 8.2.5129: timeout handling is not optimal

Problem:    Timeout handling is not optimal.
Solution:   Avoid setting timeout_flag twice.  Adjust the pointer when
            stopping the regexp timeout.  Adjust variable name.

src/regexp.c

@@ -61,6 +61,7 @@ init_regexp_timeout(long msec)
 disable_regexp_timeout(void)
 {
     stop_timeout();
+    timeout_flag = &dummy_timeout_flag;
 }
 #endif