bug#50611: one-byte (write) heap-buffer-underrun

[Jim Meyering]

Thanks for all your recent changes! I built+tested with ASAN on Fedora 34:

Configure and build as usual, then "make clean" and do this:
> san='-fsanitize-address-use-after-scope -fsanitize=address -static-libasan'; 
> ASAN_OPTIONS=detect_leaks=0 , CFLAGS='-O -ggdb3' AM_CFLAGS="$san" 
> AM_LDFLAGS="$san" check

(but that first -f option may be obsolete, because it seems to provoke
spurious failure of the stdbuf test and help-version tests)

That exposed this (and similar in an md5sum tests):

md5sum: test ck-strict-1: stderr mismatch, comparing ck-strict-1.2
(expected) and ck-strict-1.E (actual)
*** ck-strict-1.2       Wed Sep 15 17:16:39 2021
--- ck-strict-1.E       Wed Sep 15 17:16:39 2021
***************
*** 1 ****
! md5sum: WARNING: 1 line is improperly formatted
--- 1,47 ----
! =================================================================
! ==1752792==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x60c00000003f at pc 0x0000004d7387 bp 0x7fff29bac390 sp
0x7fff29bac388
! READ of size 1 at 0x60c00000003f thread T0
!     #0 0x4d7386 in digest_check src/digest.c:1076
!     #1 0x4d7386 in main src/digest.c:1492
!     #2 0x7ff1f089db74 in __libc_start_main (/lib64/libc.so.6+0x27b74)
!     #3 0x40754d in _start (/home/j/w/co/cu/src/md5sum+0x40754d)
!
! 0x60c00000003f is located 1 bytes to the left of 120-byte region
[0x60c000000040,0x60c0000000b8)
! allocated by thread T0 here:
!     #0 0x492417 in __interceptor_malloc
/home/j/w/co/gcc/libsanitizer/asan/asan_malloc_linux.cpp:129
!     #1 0x7ff1f08ec903 in _IO_getdelim (/lib64/libc.so.6+0x76903)
!     #2 0x49208f  (/home/j/w/co/cu/src/md5sum+0x49208f)
!
! SUMMARY: AddressSanitizer: heap-buffer-overflow src/digest.c:1076 in
digest_check
! Shadow bytes around the buggy address:
!   0x0c187fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
!   0x0c187fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
!   0x0c187fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
!   0x0c187fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
!   0x0c187fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
! =>0x0c187fff8000: fa fa fa fa fa fa fa[fa]00 00 00 00 00 00 00 00
!   0x0c187fff8010: 00 00 00 00 00 00 00 fa fa fa fa fa fa fa fa fa
!   0x0c187fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
!   0x0c187fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
!   0x0c187fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
!   0x0c187fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
! Shadow byte legend (one shadow byte represents 8 application bytes):
!   Addressable:           00
!   Partially addressable: 01 02 03 04 05 06 07
!   Heap left redzone:       fa
!   Freed heap region:       fd
!   Stack left redzone:      f1
!   Stack mid redzone:       f2
!   Stack right redzone:     f3
!   Stack after return:      f5
!   Stack use after scope:   f8
!   Global redzone:          f9
!   Global init order:       f6
!   Poisoned by user:        f7
!   Container overflow:      fc
!   Array cookie:            ac
!   Intra object redzone:    bb
!   ASan internal:           fe
!   Left alloca redzone:     ca
!   Right alloca redzone:    cb
! ==1752792==ABORTING