In commit af5947f4
the function btr_discard_page() is invoking btr_set_min_rec_mark()
with the wrong buf_block_t* object. node_ptr is on merge_block,
not block.

btr_discard_page(): Remove the variables merge_page, page, and
always refer to block->frame or merge_block->frame instead.
Also, limit the scope of node_ptr and avoid duplicated conditions.

btr_set_min_rec_mark(): Add a template parameter, so that the
caller can specify whether the page is supposed to have a left sibling.
Otherwise, the assertion (which was introduced in the same commit)
would fail in btr_discard_page().