fix some segfaults while using riak_get()

[folays]

• make the "not sync" operation of riak_get() really not async, thus not returing a NULL rop->response for an ERRIAK_OK status
• fix some buffer overflows into functions called by riak_print_get_response()

[bookshelfdave]

thanks @folays, we'll try to get to this PR in the next couple of days.

[hazen]

Good catches, @folays. Some of your changes might collide with #40, which is going to clean up the printing interface for all of the messages and structs. Some of the first ones which were written, like GET, are rather cumbersome as you already identified. Also we already have riak_print.c where most of the existing print routines live. We'd want to put new print routines in the same place. As @metadave mentioned, we'll look at it more carefully in the coming days.

[bookshelfdave]

@folays - would you be wiling to submit a new PR to split out the bug fixes vs riak_snprintf_cat() changes? As @javajolt mentioned, we're working on some changes that will be part of #40, which will most likely collide with yours.