test: file_md5: fix final memset 0 of md5 context
test: file_md5: fix final memset 0 of md5 context

Found using clang version 3.0-6ubuntu3

Limecraft/ebu-libmxf,stuarthicks/libmxf,Limecraft/ebu-libmxf,stuarthicks/libmxf,stuarthicks/libmxf,stuarthicks/libmxf,Limecraft/ebu-libmxf