gda-vprovider: Fix return value error
This error was caught by clang:
CC gda-vprovider-data-model.lo
/home/lantw44/gnome/source/libgda/libgda/sqlite/virtual/gda-vprovider-data-model.c:311:2: error: non-void function 'virtual_filtered_data_ref' should return a value [-Wreturn-type]
g_return_if_fail (data != NULL);
^
/home/lantw44/gnome/devinstall/include/glib-2.0/glib/gmessages.h:590:3: note: expanded from macro 'g_return_if_fail'
return; \
^
1 error generated.