make bson_realloc really free memory
use reallocf instead of realloc on OS X
Merge pull request #43 from skaes/fix-realloc-zero

make bson_realloc really free memory
memory: simplify realloc a bit.

Now that we check explicitely for num_bytes == 0, we can drop the secondary
check at the end of the realloc and simply fail on NULL.