make SBMLResults_free() ignore null pointer safely

Otherwise the following code got a segfault at the last line:
> SBMLResultsArray_t *ra;
> ra = SBMLResultsArray_allocate(1);
> SBMLResultsArray_free(ra);
This header contains libsbml's types
SBMLResultsMatrix_getResults() is not yet implemented
add unit test for sbmlsolver/sbmlResults.h