Fix out of bound access in FlatBufferBuilder::EndTable()

Detected with Clang+ASAN

Note: the rational is that ```buf_buf_.data_at(0)``` is the end of the buffer, so memcmp should not got beyond. In the case where ```*it``` is closer to 0 than ```vt1_size```, and the comparison succeeded for the ```*it``` first elements, the code was accessing out of bound.