Fix a memory leak

dex_file is meant to take ownership of `container`. There's at least
one path (`error_msg = "Invalid or truncated dex file";`) where
`container` never gets passed along to a dex_file.

Caught by the static analyzer:
art/runtime/dex/dex_file_loader.cc:406:45: warning: Potential memory
leak

Bug: None
Test: Reran the analyzer. Memory leak complaints are gone.
Change-Id: Ib57008e444d32b366bb2beabec8e39b8e84fd9db