Stop CreateUninitializedVector returning a pointer to invalid memory.

CreateUninitializedVector was performing the following actions:
    1. call StartVector.
    2. call make_space, and set buf to point to the reserved space.
    3. call EndVector.

The problem is that a call to EndVector can ultimately call make_space, which
if the buffer is full, will cause a reallocation, invalidating the value stored
in buf.  So setting buf needs to be delayed until after EndVector.

The following code, when run under valgrind shows a write to free'd memory before
the change, but no such error after:

int main()
{
    flatbuffers::FlatBufferBuilder fbb(128);
    char *buf = nullptr;
    fbb.CreateUninitializedVector(128, &buf);
    *buf = 0;
}