problem with 3D pressure solver

```
What steps will reproduce the problem?
1. run pcg program
2. run cubicrayleigh with Rayleigh_run()
3. run cubicrayleigh with Nusselt_run()

What is the expected output? What do you see instead?
None of them converged, and the while loop is dead
Message similar to:
[INFO] Sol_MultigridPressure3DBase::do_fmg - error after 3 iterations: L2 =
0.000000 (221243.654747x), Linf = 0.000000 (-nanx)
There is a "nan", which likely lead to segmentation faults or dead loop

What version of the product are you using? On what operating system?
Ubuntu 10.04LTS 64bit, CUDA 3.2, GTX460 Fermi

Please provide any additional information below.
I have noticed that two of the 18 unit tests failed due to segmentation faults
and it should be related to 3D pressure solver.
The unit test diagnostic info is as below.
******************************************************************
The following tests FAILED:
12 - MultigridMixedTest (SEGFAULT)
17 - PCGTest (SEGFAULT)
Errors while running CTest
make: *** [test] Error 8
******************************************************************
Thanks!
```

Original issue reported on code.google.com by `yush...@gmail.com` on 28 Nov 2010 at 5:03