This replaces the output parameter (which is bad API design).  As a
side effect, it fixes the bad [[gnu::pure]] attribute added by commit
a636d212 which caused optimizing compilers to miscompile calls to
that function.  "Pure" functions can be assumed to have no output
arguments, so the compiler can assume the function doesn't modify
them.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1282