Fix Ch3a uncertainty allocation problem for AVHRR/1 and AVHRR/2

[patternizer]

Global statistics suggests that data is being incorrectly stored in memory for the independent and structured uncertainties on the Ch3a of the earlier editions of the AVHRR (prior to NOAA-15).

It is suspected that this is due to incorrect initiatialisation of the Ch3a array in the AVHRR_Data type in fiduceo_uncertainties.f90.