patch 9.1.1031: Coverity complains about insecure data handling

Problem:  Coverity complains about insecure data handling
          (v9.1.1024)
Solution: use int consistently to access the blob index
          (Yegappan Lakshmanan)

related: #16468

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/strings.c b/src/strings.c
index a71ac91..53e0171 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -1265,7 +1265,7 @@
 {
     garray_T	str_ga;
     long	blen;
-    long	idx;
+    int		idx;
 
     ga_init2(&str_ga, sizeof(char), 80);