lz4: refine commit 9143a6c55e for the 64-bit case

I clearly went too far there: While the LZ4_WILDCOPY() instances indeed
need prior guarding, LZ4_SECURECOPY() needs this only in the 32-bit case
(where it simply aliases LZ4_WILDCOPY()). "cpy" can validly point
(slightly) below "op" in these cases, due to

		cpy = op + length - (STEPSIZE - 4);

where length can be as low as 0 and STEPSIZE is 8. However, instead of
removing the check via "#if !LZ4_ARCH64", refine it such that it would
also properly work in the 64-bit case, aborting decompression instead
of continuing on bogus input.

Reported-by: Mark Pryor <pryorm09@gmail.com>
Reported-by: Jeremi Piotrowski <jeremi.piotrowski@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Mark Pryor <pryorm09@gmail.com>
Tested-by: Jeremi Piotrowski <jeremi.piotrowski@gmail.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/lz4/decompress.c

@@ -147,7 +147,7 @@ static int INIT lz4_uncompress(const unsigned char *source, unsigned char *dest,
 				goto _output_error;
 			continue;
 		}
-		if (unlikely((unsigned long)cpy < (unsigned long)op))
+		if (unlikely((unsigned long)cpy < (unsigned long)op - (STEPSIZE - 4)))
 			goto _output_error;
 		LZ4_SECURECOPY(ref, op, cpy);
 		op = cpy; /* correction */
@@ -279,7 +279,7 @@ static int lz4_uncompress_unknownoutputsize(const unsigned char *source,
 				goto _output_error;
 			continue;
 		}
-		if (unlikely((unsigned long)cpy < (unsigned long)op))
+		if (unlikely((unsigned long)cpy < (unsigned long)op - (STEPSIZE - 4)))
 			goto _output_error;
 		LZ4_SECURECOPY(ref, op, cpy);
 		op = cpy; /* correction */