Pressurized soft body explodes on rigid collision when using clusters

What steps will reproduce the problem?
1. Create pressurized (m_cfg.kPR > 0) soft body
2. Set m_cfg.collisions = btSoftBody::fCollision::CL_RS
3. Generate any number of cluster (sb->generateClusters(...)). Higher number 
gives better effect.
4. Collide the soft body with a static rigid body.

This can be reproduced easily in the "Pressure" demo by adding the appropriate 
lines of code in Init_Pressure(...).

What is the expected output? What do you see instead?
Normal collision expected. Instead, the body usually explodes as soon as it 
touches the rigid body. Sometimes it becomes a bit unstable at first and 
explodes a couple seconds later (higher clusters usually makes it explode more 
quickly).

What version of the product are you using? On what operating system?
Verified in both release 2.77 and repository r2379.

Please provide any additional information below.
Turning pressure off (kPR=0) seems to prevent the explosion. The RS cluster 
solver doesn't seem to like pressure...

Original issue reported on code.google.com by dave.bru...@gmail.com on 9 Apr 2011 at 8:34

favreau / bullet

Pressurized soft body explodes on rigid collision when using clusters #504