DEV: coccinelle: Fix incorrect replacement in ist.cocci

We must not use `ist2()` if the value of `i.len` is derived from the value of
`i.ptr`:

    i.ptr = "foo";
    i.len = strlen(i.ptr);
diff --git a/dev/coccinelle/ist.cocci b/dev/coccinelle/ist.cocci
index 4945141..ea13d39 100644
--- a/dev/coccinelle/ist.cocci
+++ b/dev/coccinelle/ist.cocci
@@ -3,9 +3,14 @@
 expression p, l;
 @@
 
+(
+ i.ptr = p;
+ i.len = strlen(i.ptr);
+|
 - i.ptr = p;
 - i.len = l;
 + i = ist2(p, l);
+)
 
 @@
 @@