#84 array out of bounds in GRAB

[seismick]

Process GRAB has an array out of bounds with gfortran at line 1024,
due to the array index being zero in this compound if :

    ! SKIP TRACES THAT ARE NOT FLAGGED.
    IF (obj%hdr_flag > 0 .and. &
      HD(obj%hdr_flag,ntr_do) == 0.) CYCLE

It needs to be rewritten like this:

    IF (obj%hdr_flag > 0) then
      IF (HD(obj%hdr_flag,ntr_do) == 0.) CYCLE
END IF

This type of bug has popped up a few times. Hopefully, that is the last
of them.

[Bill Menger]

Fixed.