Minor enhancement to put_short() macro. This change saw marginal speedup
(about 0% to 3% depending on the compression level and input). I guess
the speedup likely arises from following facts:

 1) "s->pending" now is loaded once, and stored once. In the original
    implementation, it needs to be loaded and stored twice as the
    compiler isn't able to disambiguate "s->pending" and
    "s->pending_buf[]"

 2) better code generations:
   2.1) no instruction are needed for extracting two bytes from a short.
   2.2) need less registers
   2.3) stores to adjacent bytes are merged into a single store, albeit
        at the cost of penalty of potentially unaligned access.