openh264enc: fix broken sps/pps header generation

openh264enc: fix broken sps/pps header generation

This was putting a truncated SPS into the initial header instead
of the PPS because it was always reading from the beginning of the
bitstream buffer (pBsBuf) and not from the offset where the current
NAL is at in the bitstream buffer (psBsBuf + nal_offset).

This was broken in commit 17113695.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/1576

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2178>