Fix std::terminate and null pointer access in input_stream_adapter (#5699)

Parsing from a std::istream crashed in two unusual but valid stream
states, both in input_stream_adapter:

- With eofbit in the stream's exceptions() mask, get_character() sets
  eofbit via is->clear(), which throws std::ios_base::failure. While
  that exception unwinds, ~input_stream_adapter() called clear() again
  to reset eofbit, which is still set and still in the exception mask,
  so it throws a second time out of the (implicitly noexcept)
  destructor and std::terminate() is called. The destructor now only
  calls clear() if a bit other than eofbit remains set, so the first
  exception can propagate normally.
- For an std::istream without a stream buffer (rdbuf() == nullptr,
  e.g. std::istream(nullptr)), the constructor stored the null
  pointer without checking it, and get_character() dereferenced it.
  input_adapter(std::istream&) now throws parse_error.101 for such a
  stream, the same as it already does for a null FILE* or char*.

Added regression tests to unit-deserialization.cpp and, for the
JSON_PRECISE_STREAM_POSITION variant of get_character(), to
unit-precise-stream-position.cpp; both crashed before this fix.
Documented the two exceptions in parse.md and operator_gtgt.md.

Fixes #5646.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

docs/mkdocs/docs/api/operator_gtgt.md

@@ -20,7 +20,12 @@ the stream `i`
 
 ## Exceptions
 
-- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
+- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or if
+  `i` has no stream buffer (`#!cpp i.rdbuf() == nullptr`, for instance `#!cpp std::istream(nullptr)`).
+- If reading from `i` reaches the end of the input and `eofbit` is part of `i`'s
+  [`exceptions()`](https://en.cppreference.com/w/cpp/io/basic_ios/exceptions) mask, the `std::ios_base::failure`
+  thrown by `i` itself propagates instead of a `parse_error`, the same as it would for the standard library's own
+  extraction operators.
 
 ## Complexity
 
@@ -118,3 +123,5 @@ being read.
   it as end of input; planned to become the default in version 4.0.0.
 - `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave the character that terminates a number in
   the stream; planned to become the default in version 4.0.0.
+- Fixed a null pointer dereference for an `std::istream` without a stream buffer (now throws `parse_error.101`), and a
+  crash (`std::terminate`) when `i` has `eofbit` in its exception mask, in version 3.13.0.