Avoid UB in CodeItemDebugInfoAccessor::Init

Bug: http://b/219526286

Initialize `dex_file_` even if `code_item` is nullptr.  Later in
`elf_debug_writer.h:WriteCompilationUnit`, the call to
`DecodeDebugPositionInfo` assumes that `dex_file_` is initialized.

Test: Revert aosp/1984608 and the following passes:
  SOONG_ALLOW_MISSING_DEPENDENCIES=true HOST_PREFER_32_BIT=true m -j32 test-art-host-gtest-art_oatdump_tests32
Change-Id: Ie8b9adf25742a6c2448dfbbc201155b7933ef3d3
1 file changed