movenc: Check that frag_info entries exist in mov_write_sidx_tag

This fixes crashes with pathological cases when trying to write
a sidx index (via the -movflags faststart option, in combination
with fragmenting options), when no fragments actually have been
written. (This is possible if the empty_moov flag isn't used,
so that all actual packet data is written in the moov/mdat pair,
and no moof/mdat pairs have been written.)

In these pathological cases, no sidx should be written at all.

Signed-off-by: Martin Storsjö <martin@martin.st>
