Simplify the implementation of GetValueOf4Bytes

What the two different implementations currently do is simply
to write a 32 bit number, in the native endianness, into the
given buffer.

The actual purpose of this function is still unknown though,
it can be removed completely without breaking decoding - it
is possibly a remnant from earlier functionality in the
decoder.
Merge pull request #257 from mstorsjo/simplify-endian-func

Simplify the implementation of GetValueOf4Bytes