MB-8482 Fix GCC 4.7 strict aliasing warnings

src/node_types.h: In function 'decode_kv_length':
src/node_types.h:82:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
src/node_types.h: In function 'encode_kv_length':
src/node_types.h:91:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Change-Id: Idf1e91f8346056c7d1e7d66a05adaaa7485ba2f2
Reviewed-on: http://review.couchbase.org/27021
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>