gdbus-codegen: don't assert non-negativity for unsigned values

gdbus-codegen: don't assert non-negativity for unsigned values

The code generator sprinkles a few asserts in its output of the form:

g_assert (prop_id - 1 >= 0 && prop_id - 1 < %d);\n

prop_id is unsigned, though, so this generates a compiler warning for
me.

This commit changes the code to merely check for prop_id != 0 instead of
prop_id - 1 >= 0