page_cur_insert_rec_low(): When checking for common bytes with
the preceding record, exclude the header bytes of next_rec
that could have been updated by this function.

The scenario where this caused corruption was an insert of
a node pointer record. The child page number was written as
0x203 but recovered as 0x103 because the n_owned field of next_rec
was changed from 1 to 2 before the comparison was invoked.