Coverity complaint #111697: handle tiny sizes in block manager reads

250        }

   	6. Condition (__ret = __wt_buf_init(session, buf, bufsize)) != 0, taking false branch.

251        WT_RET(__wt_buf_init(session, buf, bufsize));

   	7. tainted_data_argument: Calling function __wt_read taints argument buf->mem. [show details]

   	8. Condition (__ret = __wt_read(session, block->fh, offset, size, buf->mem)) != 0, taking false branch.

252        WT_RET(__wt_read(session, block->fh, offset, size, buf->mem));

253        buf->size = size;

254

255        /*

256         * We incrementally read through the structure before doing a checksum,

257         * do little- to big-endian handling early on, and then select from the

258         * original or swapped structure as needed.

259         */

260        blk = WT_BLOCK_HEADER_REF(buf->mem);

261        __wt_block_header_byteswap_copy(blk, &swap);

   	9. Condition swap.checksum == checksum, taking true branch.

262        if (swap.checksum == checksum) {

263                blk->checksum = 0;

   	10. Condition (swap.flags & 1U) != 0, taking true branch.

CID 111697 (#1 of 1): Untrusted value as argument (TAINTED_SCALAR)

11. tainted_data: Passing tainted variable buf->mem to a tainted sink. (The function pointer resolves to __wt_checksum_sw.) [show details]