patch 8.2.3471: crash when using CTRL-T after an empty search pattern

Problem:    Crash when using CTRL-T after an empty search pattern.
Solution:   Bail out when there is no previous search pattern. (closes #8953)

src/ex_getln.c

@@ -612,7 +612,8 @@ may_adjust_incsearch_highlighting(
     // NOTE: must call restore_last_search_pattern() before returning!
     save_last_search_pattern();
 
-    if (!do_incsearch_highlighting(firstc, &search_delim, is_state, &skiplen, &patlen))
+    if (!do_incsearch_highlighting(firstc, &search_delim, is_state,
+							    &skiplen, &patlen))
     {
 	restore_last_search_pattern();
 	return OK;
@@ -626,6 +627,11 @@ may_adjust_incsearch_highlighting(
     if (search_delim == ccline.cmdbuff[skiplen])
     {
 	pat = last_search_pattern();
+	if (pat == NULL)
+	{
+	    restore_last_search_pattern();
+	    return FAIL;
+	}
 	skiplen = 0;
 	patlen = (int)STRLEN(pat);
     }

src/testdir/test_search.vim

@@ -1977,4 +1977,14 @@ func Test_pattern_is_uppercase_smartcase()
   bw!
 endfunc
 
+func Test_no_last_search_pattern()
+  CheckOption incsearch
+
+  let @/ = ""
+  set incsearch
+  " this was causing a crash
+  call feedkeys("//\x14", 'xt')
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3471,
 /**/
     3470,
 /**/