readelf: Allow .gdb_index const table to be empty

readelf: Allow .gdb_index const table to be empty

The .gdb_index const table can be empty, if no symbol table entry
refers to a name or CU. Which technically means the symbol table is
empty or has only zero members.

      * src/readelf.c (print_gdb_index_section): Check const_off is
      not past the end of the data section, it can be at the end.

Signed-off-by: Mark Wielaard <mark@klomp.org>