Fixed wrong assumption in Table::Open()

Fixed wrong assumption in Table::Open()

Summary:
`Table::Open()` assumes that `size` correctly describes the size of `file`, added a check that the footer is actually the right size and for good measure added assertions to `Footer::DecodeFrom()`.

This was discovered by running `valgrind ./db_test` and seeing that `Footer::DecodeFrom()` was accessing uninitialized memory.

Test Plan:
make clean check

ran `valgrind ./db_test` and saw DBTest.NoSpace no longer complains about a conditional jump being dependent on uninitialized memory.

Reviewers: dhruba, vamsi, emayanke, sheki

Reviewed By: dhruba

CC: leveldb
Differential Revision: https://reviews.facebook.net/D7815