fix 339721 assertion 'check_sibling == sibling' failed in readdwarf3.c ...

The skip code was wrongly skipping 16 bytes, while only 8 are read
for a DW_FORM_ref_sig8.

Note that the problem is made visible by an assert when using 
--trace-symtab=yes but in fact this is a real bug in the dwarf reader,
that was introduced in one of the optimisations done for the inline info.

It can manifest itself with other symptoms:

One of the 2 following assertions can fail:
   vg_assert (check_sibling == sibling);
   vg_assert (get_position_of_Cursor (&check_skip) 
              == get_position_of_Cursor (&c));

Or the following error can be given:
--29973-- WARNING: Serious error when reading debug info
--29973-- When reading debug info from /home/philippe/valgrind/trunk_untouched/memcheck/tests/dw4:
--29973-- Overrun whilst reading .debug_info section




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14603 a5019735-40e9-0310-863c-91ae7b9d1cf9
Indicate the fix for 339721 is only partial


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14605 a5019735-40e9-0310-863c-91ae7b9d1cf9