gobject: Sink floating pspecs if adding them to a class fails
This may fix Coverity assuming that pspecs are leaked, which is causing tens and tens of false positives in the latest Coverity reports for GLib.
Ensure that the pspecs are sunk (if floating) even if adding them to the class fails (due to validation failure or an identically named property already existing).
Signed-off-by: Philip Withnall pwithnall@endlessos.org