detect/base64_data: reset buffer offset

When in a `base64_decode`-`base64_data` pair the decode was depending
on another match through the relative option, the `buffer_offset` would
be updated to the relative position of the previous match. During the
`base64_data` phase, a relative match would use that offset even though
the match happened in a new buffer.

Example::

        http.request_body; content:"|27|";                              \
                base64_decode:relative;                                 \
                base64_data; content:"|ff ff ff ff|"; within:16;

This use of the `buffer_offset` is incorrect as that value is relative
to a buffer and the `base64_data` points to a new buffer.

This patch addresses this by resetting DetectEngineThreadCtx::buffer_offset
before inspecting `base64_data`.

Bug: OISF#7842.
(cherry picked from commit 5f92a6c)

Author: victorjulien

src/detect-base64-data.c

@@ -65,6 +65,7 @@ int DetectBase64DataDoMatch(DetectEngineCtx *de_ctx,
     DetectEngineThreadCtx *det_ctx, const Signature *s, Flow *f)
 {
     if (det_ctx->base64_decoded_len) {
+        det_ctx->buffer_offset = 0;
         return DetectEngineContentInspection(de_ctx, det_ctx, s,
             s->sm_arrays[DETECT_SM_LIST_BASE64_DATA], NULL, f, det_ctx->base64_decoded,
             det_ctx->base64_decoded_len, 0, DETECT_CI_FLAGS_SINGLE,