branches/zip: flst_write_addr(), flst_read_addr(): Assert that no

file list link is written or read below FIL_PAGE_DATA.  This has
helped diagnose issue#63.

include/fut0lst.ic

@@ -38,6 +38,8 @@ flst_write_addr(
 {
 	ut_ad(faddr && mtr);
 	ut_ad(mtr_memo_contains_page(mtr, faddr, MTR_MEMO_PAGE_X_FIX));
+	ut_a(addr.page == FIL_NULL || addr.boffset >= FIL_PAGE_DATA);
+	ut_a(ut_align_offset(faddr, UNIV_PAGE_SIZE) >= FIL_PAGE_DATA);
 
 	mlog_write_ulint(faddr + FIL_ADDR_PAGE, addr.page, MLOG_4BYTES, mtr);
 	mlog_write_ulint(faddr + FIL_ADDR_BYTE, addr.boffset,
@@ -61,6 +63,8 @@ flst_read_addr(
 	addr.page = mtr_read_ulint(faddr + FIL_ADDR_PAGE, MLOG_4BYTES, mtr);
 	addr.boffset = mtr_read_ulint(faddr + FIL_ADDR_BYTE, MLOG_2BYTES,
 				      mtr);
+	ut_a(addr.page == FIL_NULL || addr.boffset >= FIL_PAGE_DATA);
+	ut_a(ut_align_offset(faddr, UNIV_PAGE_SIZE) >= FIL_PAGE_DATA);
 	return(addr);
 }