dwz compressed alternate .debug_info and .debug_str not read correctly.

Bug #327837. The buildid from the .gnu_debugaltlink section was parsed
incorrectly (from the wrong offset). Causing the debug alt file not to
be found.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13715 a5019735-40e9-0310-863c-91ae7b9d1cf9