Fix integer overflow in DecodeEntry bounds check

[M0oo0ry]

The bounds check in DecodeEntry() compares limit - p against
*non_shared + *value_length. Both operands of the addition are
uint32_t, so the sum can wrap around when their combined value
exceeds UINT32_MAX. When that happens the comparison passes
incorrectly and the function returns a pointer that may be past
limit, which callers then dereference during block iteration.

This change widens the comparison to uint64_t so the addition cannot
wrap. Behavior on valid blocks is unchanged.

Tested:

• All ctest targets pass on Linux (leveldb_tests, c_test,
  env_posix_test).