HydroDyn does not allocate A, B, C matrices, leading to segfault in Jacobian linearization outputs

[ebranlard]

Bug description
Setting LinOutJac=True and LinOutMod=True in the main fst file will result in a segfault because the A, B ,C matrices of hydrodyn are not allocated when the number of states are 0.
This issue comes from the statements if ( p%totalStates == 0 ) return present in HydroDyn.

Potential options

1. Make sure the Jacobian functions of HydroDyn always allocate the A, B, C, D matrices. We can simply remove the "if" statements, or for some small performance gains, replace the if ( p%totalStates == 0 ) return , with some if ( p%totalStates > 0 ) then ... . This will require a reindentation of the code.

2. Surround the calls of WrPartialMatrix (in FAST_Lin) with if statement if the matrices are not allocated

3. Change WrPartialMatrix so that it supports unallocated matrices.

I'd vote for option 1 since I believe other modules like SubDyn always allocate the matrices even if they have a 0 dimension. Let me know your thoughts.

[andrew-platt]

I vote for option 1 as well.

I have a vague memory of seeing this issue and almost think I have this fixed in a branch, but can't find it now.

[ebranlard]

Removing if ( p%totalStates == 0 ) return works fine, should I just do that? That avoids the reindentation of the code.

[andrew-platt]

That might work. Looking at the allocation on line 2588, the allocation would be 0 by 1 -- we might need to double check that this works with the logic using the dXdu array in the glue code. Explicitly allocating it as 0 by 0 might be safer.