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/spellsuggest.c

@@ -1973,7 +1973,8 @@ suggest_trie_walk(
 			    sp->ts_isdiff = (newscore != 0)
 						       ? DIFF_YES : DIFF_NONE;
 			}
-			else if (sp->ts_isdiff == DIFF_INSERT)
+			else if (sp->ts_isdiff == DIFF_INSERT
+							    && sp->ts_fidx > 0)
 			    // When inserting trail bytes don't advance in the
 			    // bad word.
 			    --sp->ts_fidx;

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

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5123,
 /**/
     5122,
 /**/