db_odbc.nim some iterator has bug.

[Angluca]

> win10, nim 0.14.2

Will crash when I use fastRows and instantRows iterator. 
    
    
    echo buf, rowRes,":",cCnt, ";"
    rowRes[colId-1] = $buf.cstring # line:341 db_odbc.nim
    

result: 
    
    
    你好        nil:19;
    Traceback (most recent call last)
    db_odbc.nim(342)         temp
    SIGSEGV: Illegal storage access. (Attempt to read from nil?)
    

Crash to rowRes is nil.