patch 8.2.3438: cannot manipulate blobs

Problem:    Cannot manipulate blobs.
Solution:   Add blob2list() and list2blob(). (Yegappan Lakshmanan,
            closes #8868)

src/typval.c

@@ -470,6 +470,23 @@ check_for_opt_bool_arg(typval_T *args, int idx)
     return check_for_bool_arg(args, idx);
 }
 
+/*
+ * Give an error and return FAIL unless "args[idx]" is a blob.
+ */
+    int
+check_for_blob_arg(typval_T *args, int idx)
+{
+    if (args[idx].v_type != VAR_BLOB)
+    {
+	if (idx >= 0)
+	    semsg(_(e_blob_required_for_argument_nr), idx + 1);
+	else
+	    emsg(_(e_blob_required));
+	return FAIL;
+    }
+    return OK;
+}
+
 /*
  * Give an error and return FAIL unless "args[idx]" is a list.
  */