Describe the bug
ZSTD_seekTable_create_fromSeekable causes a segmentation fault when called with a ZSTD_seekable that is not initialized.

To Reproduce
Steps to reproduce the behavior:

1. ZSTD_seekable* const stream = ZSTD_seekable_create();
2. ZSTD_seekTable* const zst = ZSTD_seekTable_create_fromSeekable(stream);

Expected behavior
ZSTD_seekTable_create_fromSeekable returns a NULL pointer.