patch 8.2.5123: using invalid index when looking for spell suggestions

Problem:    Using invalid index when looking for spell suggestions.
Solution:   Do not decrement the index when it is zero.

src/testdir/test_spell.vim

@@ -70,6 +70,16 @@ func Test_z_equal_on_invalid_utf8_word()
   bwipe!
 endfunc
 
+func Test_z_equal_on_single_character()
+  " this was decrementing the index below zero
+  new
+  norm a0\<5C>
+  norm zW
+  norm z=
+
+  bwipe!
+endfunc
+
 " Test spellbadword() with argument
 func Test_spellbadword()
   set spell