Fix strict aliasing issue/warning

For the levels 2 and 3 of the -Wstrict-aliasing flag,
gcc (4.6.3) emitted the following warning:

src/node_types.h: In function 'decode_kv_length':
src/node_types.h:81:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Which is dangerous since by default we compile with -O2.

Change-Id: Ibffc6a4188715a7562d14bfd8d83e0843ab606d6
Reviewed-on: http://review.couchbase.org/24159
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>