Crash if custom std::streambuf implementation throws

[laudrup]

Describe the bug
Constructing an std::istream with a custom std::streambuf that throws on construction causes an access violation.

Command-line test case

C:\Temp>type foo.cpp

#include <istream>
#include <string>
#include <stdexcept>

class foo_buf : public std::streambuf {
public:
    foo_buf() {
        throw std::runtime_error("Whoops");
    }
};

class foo_stream : public std::istream {
public:
    foo_stream() : std::istream(new foo_buf) {
    }
};

int main(int, char**) {
    try {
        foo_stream f;
    } catch (const std::exception&) {
        return 1;
    }
    return 0;
}

CL.exe /c /Zi /nologo /W3 /WX- /diagnostics:column /Od /Ob0 /D WIN32 /D _WINDOWS /D "CMAKE_INTDIR=\"Debug\"" /D _MBCS /Gm- /EHsc /
  RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Zc:inline /GR /Fo"foo.dir\Debug\\" /Fd"foo.dir\Debug\vc142.pdb" /Gd /TP /errorReport:queue C:\Temp\foo.cpp  foo.cp

link.exe /ERRORREPORT:QUEUE /OUT:"C:\Temp\Debug\foo.exe" /INCREMENTAL /NOLOGO kernel32.lib user32.lib gdi32.lib wins
  pool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /manifest:embed /DEBUG /PDB:"C:/Temp/Debug/foo.pdb" /SUBSYSTEM:CONSOLE /TLBID:1 /D
  YNAMICBASE /NXCOMPAT /IMPLIB:"C:/Temp/Debug/foo.lib" /MACHINE:X64  /machine:x64 foo.dir\Debug\foo.obj

Debug\foo.exe

Exception thrown at 0x00007FFE0C6E6E85 (msvcp140d.dll) in foo.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.

Expected behavior

The std::runtime_error thrown from the foo_buf constructor should be caught and main should return 1.

STL version

• Visual Studio version

Microsoft Visual Studio Enterprise 2019
Version 16.6.5

I don't see any reason why an std::streambuf implementation shouldn't be allowed to throw and this works fine with other compilers and library implementations (Gnu and LLVM), so I assume this could be a bug in this STL implementation.

[crackedmind]

I did some debug, and found that crash happens at this line

https://github.com/microsoft/STL/blob/master/stl/src/ios.cpp#L29

because _Stdstr is not initialized at this moment and we have array out of boundary error