Fix seekable decompression in-memory api

[sean-purcell]

Issue noticed in #395 by @P-E-Meunier. I guess I never wrote examples using the in-memory API, oops. New example tests the in-memory api:

$ ./seekable_compression dickens 131072
$ ./seekable_decompression dickens.zst 2000000 2001000 | shasum
8cfd22487cd222b8756d824fa3c4419ab6c722a3  -
$ ./seekable_decompression_mem dickens.zst 2000000 2001000 | shasum
8cfd22487cd222b8756d824fa3c4419ab6c722a3  -

Same test without the patch to zstdseek_decompress.c:

$ ./seekable_decompression_mem dickens.zst 2000000 2001000 | shasum
ZSTD_seekable_init() error : An I/O error occurred when reading/seeking 
da39a3ee5e6b4b0d3255bfef95601890afd80709  -

[Cyan4973]

Thanks for jumping in @iburinoc !
Indeed, creating this test case is very helpful !

We will ensure it's run during CI tests.