Merge #11238: Add assertions before potential null deferences

Summary:
c00199244 Fix potential null dereferences (MeshCollider)

Pull request description:

Picked up by the static analyzer [Facebook Infer](http://fbinfer.com/) which I was playing around with for another research project. Just adding some asserts before dereferencing potentially null pointers.

Tree-SHA512: 9c01dab2d21bce75c7c7ef867236654ab538318a1fb39f96f09cdd2382a05be1a6b2db0a1169a94168864e82ffeae0686a383db6eba799742bdd89c37ac74397

Backport of Core PR 11238
https://github.com/bitcoin/bitcoin/pull/11238/files
Completes T627

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D2909