Problem:

A null pointer (of type mtr_t*) is de-referenced in the functions
page_cur_insert_rec_low(), page_cur_insert_rec_zip(), page_cur_delete_rec().
This problem happens only in a debug assert.

Solution:

In the debug assert, before de-referencing the pointer check if it is null.  If
the pointer is null, use some other means to verify the assert.

rb#3628 approved by Marko