Exception in SBAS MT26

An empty **self.vtec** array in **decode_sbas_iono** function causes a python3.13 runtime exception
within EGNOS bands 4 and 9 in this code
` nigp = len(self.vtec[band])
`
I have temporarily fixed it with
` -        nigp = len(self.vtec[band])
 +        try:
 +         nigp = len(self.vtec[band])
 +        except:
 +         nigp = 0
` 
but maybe there is a better solution.
