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 #3647)

Author: brammool

src/search.c

@@ -1385,7 +1385,7 @@ do_search(
 	    char_u	*trunc;
 
 	    if (*searchstr == NUL)
-		p = spats[last_idx].pat;
+		p = spats[0].pat;
 	    else
 		p = searchstr;
 	    msgbuf = alloc((unsigned)(STRLEN(p) + 40));

src/testdir/test_search.vim

@@ -1142,3 +1142,15 @@ 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

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    555,
 /**/
     554,
 /**/