read_unitinfo_dwarf2 DW_FORM_ref_addr is address size in DWARF version 2.

Bug #305513 contained a patch for some extra robustness checks. But
the real cause of crashing in the read_unitinfo_dwarf2 DWARF reader
seemed to have been this issue where DWARF version 2 DWZ partial_units
were read and DW_FORM_ref_addr had an unexpected size. This combination
is rare. DWARF version 4 is the current default version of GCC.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13367 a5019735-40e9-0310-863c-91ae7b9d1cf9