xen/unlz4: 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.

Linux commit: 2a1d689c9ba42a6066540fb221b6ecbd6298b728

Reported-by: Matthew Daley <mattjd@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/unlz4.c
+++ b/xen/common/unlz4.c
@@ -133,6 +133,7 @@ STATIC int INIT unlz4(unsigned char *inp
 			goto exit_2;
 		}
 
+		ret = -1;
 		if (flush && flush(outp, dest_len) != dest_len)
 			goto exit_2;
 		if (output)