Fix NPE in task macro accessing q"{...}".symbol.pos

We shouldn't assume that the qualifier of a `Select` is a
`SymTree`; it may be a `Block`. One place that happens
is after the transformation of named/defaults applications.
That causes the reported `NullPointerException'.

In any case, using `qual.symbol.pos` sense here; it yields the
position of the defintions *referred to* by `qual`, not the
position of `qual` itself.

Both problems are easily fixed: use `qual.pos` instead.

Fixes #1107

Review by @jsuereth