sundialsJac does not consistently check for null dstate_dt

[mcneish1]

In griddyn/solvers/sundialsInterface.cpp, sundialsJac is allowed to be passed a null dstate_dt. It acknowledges this here and here.

However,

https://github.com/LLNL/GridDyn/blob/cf850abcc921f9f1bfea917ffc03a966d64c0a6a/src/griddyn/solvers/sundialsInterface.cpp#L462

and

https://github.com/LLNL/GridDyn/blob/cf850abcc921f9f1bfea917ffc03a966d64c0a6a/src/griddyn/solvers/sundialsInterface.cpp#L471

do not check for this, and can segfault. These should have the same nullptr check as above.