patch 9.1.0810: cannot easily adjust the |:find| command

Problem:  cannot easily adjust the |:find| command
Solution: Add support for the 'findexpr' option (Yegappan Lakshmanan)

closes: #15901
closes: #15905

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/buffer.c b/src/buffer.c
index 5359527..f1e1330 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2412,6 +2412,7 @@
     clear_string_option(&buf->b_p_fp);
 #if defined(FEAT_EVAL)
     clear_string_option(&buf->b_p_fex);
+    clear_string_option(&buf->b_p_fexpr);
 #endif
 #ifdef FEAT_CRYPT
 # ifdef FEAT_SODIUM