CLEANUP: Use istadv(const struct ist, const size_t) whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    struct ist i;
    expression e;
    @@

    - i.ptr += e;
    - i.len -= e;
    + i = istadv(i, e);
diff --git a/src/cache.c b/src/cache.c
index acd6872..12609e1 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -620,8 +620,7 @@
 
 			case HTX_BLK_DATA:
 				v = htx_get_blk_value(htx, blk);
-				v.ptr += offset;
-				v.len -= offset;
+				v = istadv(v, offset);
 				if (v.len > len)
 					v.len = len;