[llvm-dwarfdump] - Fix array out of bounds access crash.

This fixes possible out of bound access in
DWARFDie::getFirstChild()
which might happen when .debug_info section is corrupted,
like shown in testcase.

Differential revision: https://reviews.llvm.org/D39185


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316566 91177308-0d34-0410-b5e6-96231b3b80d8