Crash during read()/xsgetn() on default constructed ifstream

[rupprecht]

After #165223, and including the fix in #167779, the following crashes during the read() call:

#include <cassert>
#include <fstream>

int main() {
    char buf[10];
    std::ifstream ifs;

    assert(!ifs.is_open());
    assert(!ifs.bad());
    assert(!ifs.fail());
    assert(!ifs.eof());
    assert(ifs.good());

    ifs.read(buf, sizeof(buf));  // Fails here

    assert(!ifs.is_open());
    assert(!ifs.bad());
    assert(ifs.fail());
    assert(ifs.eof());
    assert(!ifs.good());
}

I couldn't get a stack trace w/o using asan, so here's the asan stack trace:

==1==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7d6b3b67fb52 bp 0x7ffde2c5ccc0 sp 0x7ffde2c5c430 T0)
==1==The signal is caused by a READ memory access.
==1==Hint: address points to the zero page.
    #0 0x7d6b3b67fb52 in _IO_fread (/lib/x86_64-linux-gnu/libc.so.6+0x7fb52) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)
    #1 0x5f78b3e77a19 in fread /root/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc
    #2 0x7d6b3bbe6cd6 in std::__1::basic_filebuf<char, std::__1::char_traits<char>>::xsgetn(char*, long) (/opt/compiler-explorer/clang-assertions-trunk/lib/x86_64-unknown-linux-gnu/libc++.so.1+0x77cd6)
    #3 0x7d6b3bbdcd6f in std::__1::basic_istream<char, std::__1::char_traits<char>>::read(char*, long) (/opt/compiler-explorer/clang-assertions-trunk/lib/x86_64-unknown-linux-gnu/libc++.so.1+0x6dd6f)
    #4 0x5f78b3f410ad in main /app/example.cpp:14:9
    #5 0x7d6b3b629d8f  (/lib/x86_64-linux-gnu/libc.so.6+0x29d8f) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)
    #6 0x7d6b3b629e3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e3f) (BuildId: d5197096f709801829b118af1b7cf6631efa2dcd)
    #7 0x5f78b3e573e4 in _start (/app/output.s+0x2e3e4)

Live repro: https://compiler-explorer.com/z/3vxqM7ssz

[rupprecht]

For completeness, here are two more failures related to this commit. I suspect there is one fix that will take care of everything.

1. unget() doesn't work (note: doesn't crash though):

assert(!ifs.fail());
ifs.read(buf, sizeof(buf));
ifs.unget();
assert(!ifs.fail());  // This fails now

Note: this issue exists here: https://github.com/MiniZinc/libminizinc/blob/09a95c0d19f0430f5d34679cd8a208012d417de2/lib/json_parser.cpp#L310

Live repro: https://compiler-explorer.com/z/bEn8evrhP

---

2. reading after close when using rdbuf now crashes:

std::ifstream ifs("xyz");
std::cin.rdbuf(ifs.rdbuf());
std::cin.read(buf, sizeof(buf));
ifs.close();
std::cin.read(buf, sizeof(buf));  // Fails here

Note: this issue exists here: https://github.com/apache/arrow/blob/1f8cec331ecf757ac58d41e8915db0860f1de29d/cpp/src/arrow/io/file_test.cc#L1148

Live repro: https://compiler-explorer.com/z/v5M11rz73