dx, dy can be uninitialised in index file

[nichannah]

I have found that dx, dy in the index file can be non-deterministic and sometimes incorrect in the index file. For example, here is the output from 2 consecutive runs:

dx = 0.000000e+00
dy = inf

dx = 0.000000e+00
dy = 3.193589e+04

I think the reason for this is that the the line https://github.com/openwfm/convert_geotiff/blob/master/read_geotiff.c#L65 can fail and since idx,dx and idx.dy are not initialised they take on random values. If these values are < 0 then everything is OK because there is code that catches this later on.