c_glib service method should return result from handler method

Generated c_glib service method has gboolean return type but actually returned nothing at all.

It seems that while gcc 4.9.2 treats it as if returning non-zero, clang 3.6.0 treats it as if returning zero.

The patch adds missing "return " to implementation(handler) method call in the last line of service method.

Note that it also includes style fix (0 -> FALSE) in the same line which is not essential.