patch 8.2.3582: reading uninitialized memory when giving spell suggestions

Problem:    Reading uninitialized memory when giving spell suggestions.
Solution:   Check that preword is not empty.

src/spellsuggest.c

@@ -1619,7 +1619,7 @@ suggest_trie_walk(
 		    // char, e.g., "thes," -> "these".
 		    p = fword + sp->ts_fidx;
 		    MB_PTR_BACK(fword, p);
-		    if (!spell_iswordp(p, curwin))
+		    if (!spell_iswordp(p, curwin) && *preword != NUL)
 		    {
 			p = preword + STRLEN(preword);
 			MB_PTR_BACK(preword, p);

src/testdir/test_spell.vim

@@ -839,6 +839,14 @@ func Test_spell_screendump()
   call delete('XtestSpell')
 endfunc
 
+func Test_spell_single_word()
+  new
+  silent! norm 0R00
+  spell! <20><>
+  silent 0norm 0r$ Dvz=
+  bwipe!
+endfunc
+
 let g:test_data_aff1 = [
       \"SET ISO8859-1",
       \"TRY esianrtolcdugmphbyfvkwjkqxz-\xEB\xE9\xE8\xEA\xEF\xEE\xE4\xE0\xE2\xF6\xFC\xFB'ESIANRTOLCDUGMPHBYFVKWJKQXZ",

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3582,
 /**/
     3581,
 /**/