Advance input pointer when LZ4 compressing data

Advance input pointer when LZ4 compressing data

LZ4File_write() did not advance the input pointer on subsequent invocations of
LZ4F_compressUpdate(). As a result the generated compressed output would be a
compressed version of the same input chunk.

Tests failed to catch this error because the data would comfortably fit
within the default buffer size, as a single chunk. Tests have been added
to provide adequate coverage of multi-chunk compression.

WriteDataToArchiveLZ4() which is also using LZ4F_compressUpdate() did
not suffer from this omission.

Author: Georgios Kokolatos <gkokolatos@pm.me>
Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/ZFhCyn4Gm2eu60rB%40paquier.xyz