Code cleanup: Refactor abbrev_table_read_table cycle
    
    I find it as an improvement on its own, it prevents more code duplication in
    a future patch.
    
    gdb/ChangeLog
    2017-02-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
    
    	* dwarf2read.c (abbrev_table_read_table): Read the data only once.