btr_cur_pessimistic_insert(): Remove local variable "block".  Replace
an extraneous call to btr_cur_position() [with wrong "block", in r948]
with a debug assertion that the cursor is positioned on the preceding
record after the page split.

btr_pcur_restore_position(): Do not use btr_pcur_get_page(), because
the debug assertion there could fail.  This bug was introduced in r928.