bpo-18374: fix wrong col_offset of some ast.BinOp instances (GH-14607)

Nested BinOp instances (e.g. a+b+c) had a wrong col_offset for the
second BinOp (e.g. 2 instead of 0 in the example). Fix it by using the
correct st node to copy the line and col_offset from in ast.c.