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;
       }

src/nvim/testdir/test_search.vim

@@ -982,6 +982,18 @@ func Test_search_sentence()
   /
 endfunc
 
+" Test that there is no crash when there is a last search pattern but no last
+" substitute pattern.
+func Test_no_last_substitute_pat()
+  " Use viminfo to set the last search pattern to a string and make the last
+  " substitute pattern the most recent used and make it empty (NULL).
+  call writefile(['~MSle0/bar', '~MSle0~&'], 'Xviminfo')
+  rviminfo! Xviminfo
+  call assert_fails('normal n', 'E35:')
+
+  call delete('Xviminfo')
+endfunc
+
 func Test_large_hex_chars1()
   " This used to cause a crash, the character becomes an NFA state.
   try