patch 8.2.0039: memory access error when "z=" has no suggestions

Problem:    Memory access error when "z=" has no suggestions.
Solution:   Check for negative index.

src/spellsuggest.c

@@ -3729,7 +3729,8 @@ cleanup_suggestions(
 	for (i = keep; i < gap->ga_len; ++i)
 	    vim_free(stp[i].st_word);
 	gap->ga_len = keep;
-	return stp[keep - 1].st_score;
+	if (keep >= 1)
+	    return stp[keep - 1].st_score;
     }
     return maxscore;
 }

src/testdir/test_spell.vim

@@ -241,9 +241,7 @@ func Test_spellsuggest_option_number()
   \ .. "Type number and <Enter> or click with mouse (empty cancels): ", a)
 
   set spell spellsuggest=0
-  " FIXME: the following line is currently commented out as it triggers a
-  " memory error detected in cleanup_suggestions() by asan or valgrind.
-  "call assert_equal("\nSorry, no suggestions", execute('norm z='))
+  call assert_equal("\nSorry, no suggestions", execute('norm z='))
 
   " Unlike z=, function spellsuggest(...) should not be affected by the
   " max number of suggestions (2) set by the 'spellsuggest' option.

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    39,
 /**/
     38,
 /**/