Fixed error return condition when the toc is read.

Prior to this `getToc` could return (nil, nil) which caused a crash
later.

Test: m extract_apks && out/host/linux-x86/bin/extract_apks \
        -o foo.apks -sdk-version=33 \
        -abis=X86_64,X86,ARM64_V8A,ARMEABI_V7A \
        -screen-densities=all -extract-single test.apks
  where test.apks comes from ag/19673938
 Test: vendor/google/modules/ArtGoogle/build-with-prebuilt-art-module.sh droid
  with prebuilts in place for cf_x86_64_phone_prebuilt_art-userdebug
  chained build on git_master_throttled
Bug: 246562532
Change-Id: Id849371b4dea04f08c07b201c8a466735b5b0994