MDEV-34921 MemorySanitizer reports errors for non-debug builds

my_b_encr_write(): Initialize also block_length, and at the same time
last_block_length, so that all 128 bits can be initialized with fewer
writes. This fixes an error that was caught in the test
encryption.tempfiles_encrypted.

test_my_safe_print_str(): Skip a test that would attempt to
display uninitialized data in the test unit.stacktrace.
Previously, our CI did not build unit tests with MemorySanitizer.

handle_delayed_insert(): Remove a redundant call to pthread_exit(0),
which would for some reason cause MemorySanitizer in clang-19 to
report a stack overflow in a RelWithDebInfo build. This fixes a
failure of several tests.

Reviewed by: Vladislav Vaintroub

sql/sql_insert.cc

@@ -3498,7 +3498,6 @@ pthread_handler_t handle_delayed_insert(void *arg)
     DBUG_LEAVE;
   }
   my_thread_end();
-  pthread_exit(0);
 
   return 0;
 }