EOF and query crashed caused by invalid index in columnFromVar, called from qualFromBoolExpr

[kaidaguerre]

columnFromVar tries to determine the index of the type information in the cinfos array using

var arrayIndex = int(variable.varattno - 1)

However in some cases varattno is negative, causing unpredictable results

The tactical solution is to put a range check on the index.

More work is needed to understand why this value is negative and identify potential bugs further upstream
#186