This shows up as an assertion failure when sprintf is called with
a specifier like "%.8g" and libquadmath is linked in:

Fatal glibc error: printf_buffer_as_file.c:31
  (__printf_buffer_as_file_commit): assertion failed:
  file->stream._IO_write_ptr <= file->next->write_end

Fix this by detecting pointer wraparound in __vsprintf_internal
and saturate the addition to the end of the address space instead.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

Always null-terminate the buffer and set E2BIG if the buffer is too
small.  This fixes bug 27857.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

This fixes bug 27124 because the problematic built-in vtable is gone.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

The internal buffer size is set to 2048 bytes.  This is less than
the original BUFSIZ value used by buffered_vfprintf before
the conversion, but it hopefully covers all cases where write
boundaries matter.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

The buffer resizing algorithm is slightly different.  The initial
buffer is on the stack, and small buffers are directly allocated
on the heap using the exact required size.  The overhead of the
additional copy is compensated by the lowered setup cost for buffers
compared to libio streams.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>