DBD-PostgreSQL PGTableColumnInfo>>initializeFrom: crash scenario

[Mark Bratcher]

The `initializeFrom:` selector for the `PGTableColumnInfo` class doesn't 
check for nil values for the `prec` and `scale` values. Therefore, under 
some conditions, the following will crash:

    initializeFrom: aRow [
        | prec radix scale |
        name := aRow atIndex: 1.
        type := aRow atIndex: 2.
        size := aRow atIndex: 3.
        prec := aRow atIndex: 4.
        radix := aRow atIndex: 5.
        scale := aRow atIndex: 6.
        nullable := (aRow atIndex: 7) = 'YES'.
        index := aRow atIndex: 8.

        radix = 2 ifTrue: [             "if radix is nil, this crashes
            prec := (prec / 3.32192809) ceiling."if prec is nil, this 
crashes"
            scale := (scale / 3.32192809) ceiling ]."if scale is nil, 
this crashes"

This method needs to check wither there are actually values available at 
`aRow` at indices 4, 5, and 6. Other indices might also need to be 
checked, but I have not encountered these as faults (yet).