Protected_scm warning: returning reference to temporary

[Dan Eble]

I noticed these errors building staging in lilydev 3.0:

> /home/dan/lilypond-git/lily/protected-scm.cc: In member function 
> 'Protected_scm::operator scm_unused_struct* const&() const':
> /home/dan/lilypond-git/lily/protected-scm.cc:75:56: warning: returning 
> reference to temporary [enabled by default]
> /home/dan/lilypond-git/lily/protected-scm.cc: In member function 
> 'Protected_scm& Protected_scm::operator=(const Protected_scm&)':
> /home/dan/lilypond-git/lily/protected-scm.cc:60:16: warning: '<anonymous>' 
> may be used uninitialized in this function [-Wuninitialized]
> /home/dan/lilypond-git/lily/protected-scm.cc:75:56: note: '<anonymous>' was 
> declared here
> ./out/parser.cc: In function 'int yyparse(Lily_parser*, scm_unused_struct**)':
> ./out/parser.cc:2999:12: warning: conversion to 'yytype_int16 {aka short 
> int}' from 'int' may alter its value [-Wconversion]

— 
Dan