Zero offsets are non-sensical in FlatBuffers (since offsets are
relative to themselves) but were allowed by the verifier. This could
cause buffers made up of all zeroes to be interpreted as correct
buffers with an empty root object.

Generally, not allowing such offsets will make the verifier more
likely to catch problems earlier.

Change-Id: I54010bea29721b326ff8e5348fcd9fe78e5e7506
Tested: on Linux.