Fix segmentation fault in test_tidstore.

Fix segmentation fault in test_tidstore.

The do_set_block_offsets() and other functions accessing the tidstore
did not check if the tidstore was NULL. This led to a segmentation
fault when these functions are called without calling the
test_create().

This commit adds NULL checks in relevant functions of test_tidstore to
raise an error instead if the tidstore is not initialized.

Bug: #18483
Reported-by: Alexander Kozhemyakin
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/18483-30bfff42de238000%40postgresql.org