glib-genmarshal: Avoid a param ref for static param valist arguments
When building a valist marshaller, we can avoid reffing a GParamSpec
if the argument is known to always be static. The marshaller we ship in
gmarshal.c
got this right, but marshallers generated by
glib-genmarshal were missing the optimisation. Fix that, and add a unit
test.
Signed-off-by: Philip Withnall withnall@endlessm.com
Fixes: #1792 (closed)