core: Don't use deallocated memory
core: Don't use deallocated memory

When authenticating asynchronously, don't attempt to use the username and
password from the asynchronous closure after it's been implicitly deallocated
by changing the result value of the GSimpleAsyncResult.

See: bgo#635736

Pecisk/libgdata,pwithnall/libgdata,GNOME/libgdata,Pecisk/libgdata-gtasks,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata,Distrotech/libgdata,Pecisk/libgdata,Pecisk/libgdata-gtasks,pwithnall/libgdata,Distrotech/libgdata