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.
diff --git a/src/spellsuggest.c b/src/spellsuggest.c
index 5b460a3..8f97565 100644
--- a/src/spellsuggest.c
+++ b/src/spellsuggest.c
@@ -1973,7 +1973,8 @@
 			    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;
diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index aa57444..0fd5ed9 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -70,6 +70,16 @@
   bwipe!
 endfunc
 
+func Test_z_equal_on_single_character()
+  " this was decrementing the index below zero
+  new
+  norm a0\Ê
+  norm zW
+  norm z=
+
+  bwipe!
+endfunc
+
 " Test spellbadword() with argument
 func Test_spellbadword()
   set spell
diff --git a/src/version.c b/src/version.c
index 5411e1c..0a42274 100644
--- a/src/version.c
+++ b/src/version.c
@@ -735,6 +735,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    5123,
+/**/
     5122,
 /**/
     5121,