Fix gdb.Value.dynamic_type attribute

gdb currently crashes if you try to get the dynamic_type from a
gdb.Value of a POD struct:

(gdb) py print(gdb.parse_and_eval('pod').dynamic_type)
Fatal signal: Segmentation fault

It happens because value_rtti_type() returns NULL for them, and this is
not handled correctly.

Fixed by using val->type() as a fallback in this case.

Approved-By: Simon Marchi <simon.marchi@efficios.com>