rd_grav_survey_assert_RPORV has infinite loop if no init_porv > 0

[eivindjahren]

The problem is that it will try to check 100 random non-zero points, but there may be fewer than 100 non-zero points in which case it is just an infinite loop:

resdata/lib/resdata/rd_grav.cpp

Line 376 in c76fecd

int check_points = 100;