lib/decompress_unlz4.c: always set an error return code on failures

lib/decompress_unlz4.c: always set an error return code on failures

"ret", being set to -1 early on, gets cleared by the first invocation of
lz4_decompress()/lz4_decompress_unknownoutputsize(), and hence subsequent
failures wouldn't be noticed by the caller without setting it back to -1
right after those calls.

Reported-by: Matthew Daley <[email protected]>
Signed-off-by: Jan Beulich <[email protected]>
Cc: Kyungsik Lee <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>