generator: avoid double lookup in dictionary when DeAlias is called

We can avoid a double lookup in the types dictionary here because the
method TryGetValue() was just called before.
generator: fix possible unhandled KeyNotFoundException (regression)

This regression surfaced after 82a957bc9d3d1fcc44c715a1020fe2afd16c7f4a [1],
when many collections were migrated to generic collections. HashTables
simply return null when queried for certain key, while Dictionary objects
throw KeyNotFoundException.

The regression could be noticed, more particularly, when trying to
compile the gtk3 branch of the gudevsharp project [2].

[1] https://github.com/mono/gtk-sharp/commit/82a957bc9d3d1fcc44c715a1020fe2afd16c7f4a
[2] https://github.com/mono/gudev-sharp/commits/gtk3
Merge pull request #64 from knocte/master

generator: fix possible unhandled KeyNotFoundException (regression)