vim-patch:8.1.0555: crash when last search pat is set but not last substitute pat

Problem:    Crash when last search pat is set but not last substitute pat.
Solution:   Do not mix up last search pattern and last subtitute pattern.
            (closes vim/vim#3647)
2fb8f684d8

src/nvim/search.c

@@ -1179,7 +1179,7 @@ int do_search(
       }
 
       if (*searchstr == NUL) {
-        p = spats[last_idx].pat;
+        p = spats[0].pat;
       } else {
         p = searchstr;
       }