lyrics: replaced asserts with error handler

lyrics: replaced asserts with error handler

Handle missing artist and/or title for lyrics by defining an empty list of
plugins that fails almost immediately. Note that args in lyrics_load may now
start with NULL as the first array element. make_argv() in plugin.c made the
assumption that at least one argument is passed. If the first argument is
NULL a bogus pointer is returned by make_argv(). By converting do {} while
into while {}, we allow the first argument to be NULL.