Blind attempt to fix pglz_maximum_compressed_size

Commit 11a078c triggered failures on big-endian machines, and the
only plausible place for an issue seems to be that TOAST_COMPRESS_SIZE
calls VARSIZE instead of VARSIZE_ANY. So try fixing that blindly.

Discussion: https://www.postgresql.org/message-id/20191001131803.j6uin7nho7t6vxzy%40development

Author: tvondra

src/include/access/toast_internals.h

@@ -31,7 +31,7 @@ typedef struct toast_compress_header
  */
 #define TOAST_COMPRESS_HDRSZ		((int32) sizeof(toast_compress_header))
 #define TOAST_COMPRESS_RAWSIZE(ptr) (((toast_compress_header *) (ptr))->rawsize)
-#define TOAST_COMPRESS_SIZE(ptr)	((int32) VARSIZE(ptr) - TOAST_COMPRESS_HDRSZ)
+#define TOAST_COMPRESS_SIZE(ptr)	((int32) VARSIZE_ANY(ptr) - TOAST_COMPRESS_HDRSZ)
 #define TOAST_COMPRESS_RAWDATA(ptr) \
 	(((char *) (ptr)) + TOAST_COMPRESS_HDRSZ)
 #define TOAST_COMPRESS_SET_RAWSIZE(ptr, len) \