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.
diff --git a/src/regexp.c b/src/regexp.c
index 0a6a8af..32661f6 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -61,6 +61,7 @@
 disable_regexp_timeout(void)
 {
     stop_timeout();
+    timeout_flag = &dummy_timeout_flag;
 }
 #endif