ResdataGrav.add_survey_FIP will segfault if add_std_density is not called for corresponding pvtnum

[eivindjahren]

The following works fine

            init = ResdataFile(init_file)

            grav = ResdataGrav(grid, init)

            restart_file = ResdataFile(unrst_file)
            restart_view = restart_file.restartView(sim_time=datetime.date(2000, 1, 1))

            grav.add_std_density(1, 0, 0.5)
            grav.add_survey_FIP("fip", restart_view)

however, omitting add_std_density will segfault on the following line:

resdata/lib/resdata/rd_grav.cpp

Line 220 in 5f95e31

grav_phase->fluid_mass[iactive] = fip * std_density[pvtnum];