patch 9.1.1131: potential out-of-memory issue in search.c

patch 9.1.1131: potential out-of-memory issue in search.c

Problem:  potential out-of-memory issue in search.c
Solution: improve situation and refactor search.c slightly
          (John Marriott)

- In function update_search_stat():
  add a check for a theoretical null pointer reference, set and remember
  the length of lastpat, remove the three calls to STRLEN() and use the
  various string's associated lengths instead, add a check for an
  out-of-memory condition.

- In function search_for_fuzz_match():
  remove a call to strnsave() and thus avoid having to add a check for
  an out-of-memory condition, also replace the call to STRLEN() by
  ml_get_buf_len().

closes: #16689

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>