follow up to fix for 339721 assertion 'check_sibling == sibling' failed in readdwarf3...

follow up to fix for 339721 assertion 'check_sibling == sibling' failed in readdwarf3.c ...

The fix committed in revision 14603 is properly fixing the bug 339721.
However, when enabling the dwarf tracing, the DW_FORM_ref_sig8 causes
a segmentation violation, as the tracing code is shared with the
reading code. But the DW_FORM_ref_sig8 reading code is dereferencing
some data structure that is only initialised when --read-var-info=yes.

So, in case DW_FORM_ref_sig8 form reading is done and --read-var-info=no,
then check that we are tracing, and avoid dereferencing the (not initialised)
signature hash table.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14610