Make sure all arrays are initialized before use

[mnlevy1981]

@mvertens reported an error while testing POP's NUOPC driver that manifest itself as float point errors in compute_scavenge() that she traced back to uninitialized values in dust%hflux_in. This seemed very similar to what @klindsay28 saw back in 2018. Taking my own advice from back then, I ran the call_compute_subroutines with the intel compiler using the -g -O0 -init=snan,arrays compile flags (among others). Sure enough, the tracebacks were pointing to the same line of compute_scavenge() that was causing problems in the NUOPC tests.