This is a corner case that will most likely never occur in practice,
but we have several testcases that compress and decompress zero sized
data. In that case during decompression we might malloc a buffer of
size zero. It is allowed for malloc to return NULL in that case. But
we do need a non-NULL buffer to return and set as result. So make sure
to always at least allocate one byte. Also make sure that we don't
allocate a zero sized conversion buffer for the data.