fix #942 : streaming interface does not compress after ZSTD_initCStream()

fix #942 : streaming interface does not compress after ZSTD_initCStream()

While the final result is still, technically, a frame,
the resulting frame expands initial data instead of compressing it.
This is because the streaming API creates a tiny 1-byte buffer for input,
because it believes input is empty (0-bytes),
because in the past, 0 used to mean "unknown" instead.

This patch fixes the issue.
Todo : add a test which traps the issue.