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.
diff --git a/src/spellsuggest.c b/src/spellsuggest.c
index 9d6df79..adf090b 100644
--- a/src/spellsuggest.c
+++ b/src/spellsuggest.c
@@ -3729,7 +3729,8 @@
 	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;
 }