Drop transitions in breadcrumb_builder if it's parent has been garbage collected

[ghost-not-in-the-shell]

We recently have seen that multiple user nodes crashed with "dangling mask error" (#6731) which is caused by the following scenario:

1. A node boots up and finds the network is relatively close to its best tip so it tries to do catchup.
2. Before it finishes catchup, the network moves forward and causes the node to do bootstrap.
3. The catchup process is still working on the old frontier which has been garbage collected.

To be specific, here:

mina/src/lib/transition_handler/breadcrumb_builder.ml

Line 97 in 0c6c242

Transition_frontier.Breadcrumb.build ~logger

The call to Breadcrumb.build assumes that parent is still valid.

One option is to move the check closer to Breadcrumb.build function, another option would be change the assertion to an exception and catch the exception in Breadcrumb_builder.