Remove useless LZ4 system call on failure when writing file header

Remove useless LZ4 system call on failure when writing file header

If an error occurs when writing the LZ4 file header, LZ4F_compressEnd()
was called in the error code path of write(), followed by
LZ4F_freeCompressionContext() to finish the cleanup.  The code as-is was
not broken, but the LZ4F_compressEnd() proves to not be necessary as
there are no contents to flush at this stage, so remove it.

Per gripe from Jeevan Ladhe and Robert Haas.

Discussion: https://postgr.es/m/CAOgcT0PE33wbD7giAT1OSkNJt=p-vu8huq++qh=ny9O=SCP5aA@mail.gmail.com