The wrong variable was used.  This has been broken by commit
4eeea640 seven years ago - and nobody as noticed, d'oh!

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

Found with constVariable
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with bugprone-incorrect-roundings
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-make-unique
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

The scan methods must be "noexcept".

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The libWavPack documentation says:

 "After a FALSE return the file should not be accessed again (other
 than to close it); this is a fatal error."