reftable/block: fix OOB read with bogus restart offset

reftable/block: fix OOB read with bogus restart offset

Restart points encode records in a given block that do not use prefix
compression and that can thus immediately be seeked to. These offsets
are encoded in the restart table, where each offset needs to point at
one of the records of the block. We do not verify this though, so a
bogus restart offset may cause an out-of-bounds read:

  ==1472280==ERROR: AddressSanitizer: SEGV on unknown address 0x7d8ff7de5f7f (pc 0x55555599502b bp 0x7fffffff4df0 sp 0x7fffffff4d40 T0)
  ==1472280==The signal is caused by a READ memory access.
      #0 0x55555599502b in get_var_int ./git/build/../reftable/record.c:30:6
      #1 0x555555995c2a in reftable_decode_keylen ./git/build/../reftable/record.c:177:6
      #2 0x55555598e85c in restart_needle_less ./git/build/../reftable/block.c:455:6
      #3 0x55555598895f in binsearch ./git/build/../reftable/basics.c:175:9
      #4 0x55555598e189 in block_iter_seek_key ./git/build/../reftable/block.c:543:6
      #5 0x555555814aee in test_reftable_block__corrupt_restart_offset ./git/build/../t/unit-tests/u-reftable-block.c:636:20
      #6 0x5555557f684e in clar_run_test ./git/build/../t/unit-tests/clar/clar.c:335:3
      #7 0x5555557f2e69 in clar_run_suite ./git/build/../t/unit-tests/clar/clar.c:431:3
      #8 0x5555557f2882 in clar_test_run ./git/build/../t/unit-tests/clar/clar.c:636:4
      #9 0x5555557f375f in clar_test ./git/build/../t/unit-tests/clar/clar.c:687:11
      #10 0x5555557fa49d in cmd_main ./git/build/../t/unit-tests/unit-test.c:62:8
      #11 0x55555584c25a in main ./git/build/../common-main.c:9:11
      #12 0x7ffff7a2b284 in __libc_start_call_main (/nix/store/57iz36553175g3178pvxjij8z5rcsd4n-glibc-2.42-61/lib/libc.so.6+0x2b284) (BuildId: 8ae0b698f2d4e727f569f64bb166e08ae30bd077)
      #13 0x7ffff7a2b337 in __libc_start_main@GLIBC_2.2.5 (/nix/store/57iz36553175g3178pvxjij8z5rcsd4n-glibc-2.42-61/lib/libc.so.6+0x2b337) (BuildId: 8ae0b698f2d4e727f569f64bb166e08ae30bd077)
      #14 0x555555694c24 in _start (./git/build/t/unit-tests+0x140c24)

  ==1472280==Register values:
  rax = 0x00007d8ff7de5f7f  rbx = 0x00007fffffff4e00  rcx = 0x00007d8ff7de5f80  rdx = 0x00007bfff5b6af60
  rdi = 0x00007bfff5b6af40  rsi = 0x00007bfff592dfa0  rbp = 0x00007fffffff4df0  rsp = 0x00007fffffff4d40
   r8 = 0x00000000ff00002b   r9 = 0x00007d8ff7de5f7f  r10 = 0x00000f7ffeb25bf0  r11 = 0xf3f30000f1f1f1f1
  r12 = 0x00007fffffff58f8  r13 = 0x0000000000000001  r14 = 0x00007ffff7ffd000  r15 = 0x0000555556055fd0
  AddressSanitizer can not provide additional info.
  SUMMARY: AddressSanitizer: SEGV ./git/build/../reftable/record.c:30:6 in get_var_int

Guard against such restart offsets and signal an error to the caller via
`args.error`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>