Fix T80899: Crash on editing multiple UVs of multiple different objects at the same time

The issue was two fold.

First something sets the loop element tag and doesn't clear it before
the UV code in question tries to use the tags. Added a sanity clear to
make sure that it operates on a clean tag state.

The next one was that the UV maps in question had quite a few points
that had zero length UV loop edges. This would lead to division by
zero.

Reviewed By: Jeroen Bakker, Brecht

Differential Revision: http://developer.blender.org/D8967

Fix T80899: Crash on editing multiple UVs of multiple different objects at the same time

The issue was two fold.

First something sets the loop element tag and doesn't clear it before
the UV code in question tries to use the tags. Added a sanity clear to
make sure that it operates on a clean tag state.

The next one was that the UV maps in question had quite a few points
that had zero length UV loop edges. This would lead to division by
zero.

Reviewed By: Jeroen Bakker, Brecht

Differential Revision: http://developer.blender.org/D8967

Fix T80651: Material tab disappears

Always return true if the object can have materials. Even if there are
not currently any materials assigned to the object.

Fix material null pointer reference in buttons_context.c

ASAN reported null pointer access when converting a mesh to curves.
Check if the material is a null pointer before trying to use it.

Fix uvsculpt null pointer reference in paint.c

ASAN reported null pointer access when going into edit mode on the
default cube.

Check in uvculpt has been initialized before trying to use it.

Add nullptr checks in the depsgraph physics relation builder

Without these check ASAN complains about null pointer member access.

Reviewed By: Sergey

Differential Revision: http://developer.blender.org/D8847

Fix T80596: Convert to Curve from Mesh crashes Blender

The point cache code needs a non NULL rbw pointer.

This could have been avoided if there was a sanity check in the convert
function, so added a check there as well.

Cleanup: Fix clang-tidy warning in the new boolean code

No functional changes.

Fix building on windows after cleanup changes in tiff.c

Cleanup: Remove redundant redeclaration in collada

No functional changes.