Ensure we ignore superfluous data after dex container.

Plain `.dex` file shall contain just single container.

This ensures GetMultiDexChecksums() matches Open().

Drive by fix.  There is no known bug caused by this.

Bug: 455778210
Test: art_libdexfile_tests
Flag: EXEMPT BUGFIX
Change-Id: I8badb28dddc1cef689a229c3d00263bf0ada71aa