asn1helpers: Add new OCTET_STRING_noalloc

This assings the buffer + size of an OCTET_STRING without allocating a new
buffer and copying the data.
diff --git a/src/asn1helpers.h b/src/asn1helpers.h
index 0d121bb..007f207 100644
--- a/src/asn1helpers.h
+++ b/src/asn1helpers.h
@@ -18,3 +18,9 @@
 uint32_t asn1bitstr_to_u32(const BIT_STRING_t *in);
 uint32_t asn1bitstr_to_u28(const BIT_STRING_t *in);
 uint32_t asn1bitstr_to_u24(const BIT_STRING_t *in);
+
+static inline void OCTET_STRING_noalloc(OCTET_STRING_t *s, const uint8_t *str, int size)
+{
+	s->buf = str;
+	s->size = size;
+}