driver: slbt_init_link_params(): explicitly initialize the 'prefix' variable.
    
    This addresses a compiler false positive identification of a missing
    variable initialization.
    
    Note that access to 'prefix' is preceded by the following check:
    
    if (!libname)
    	return 0;
    
    and that all code paths which include an initialization of 'libname'
    also entail the initialization of 'prefix'.