conv: Endless while-loop when idec or jdec=0
If idec or jdec is accidentally set to 0, the program doesn't terminate as idec/jdec is used as loop increment.
Would be good if theses values are caught before entering the while-loop.
Example (will run forever):
conv 1 0 filters/gauss5x5 input.PRM test.grd
This issue might be related to issue #1064 (https://gmt.soest.hawaii.edu/issues/1064) as intf_tops.csh calls filter.csh which passes sometimes 0 values as idec jdec to conv (idec/jdec are set by integer division)
[Version: 5.x Compiled after commit r463 (2017-05-30)]