ovs-lib: make db_get_map return empty dict on error
ovs-lib: make db_get_map return empty dict on error

bug 1050504

this fixes a crash caused when we try to iterate over the return value
of db_get_map

Change-Id: I56640035c3e166ddcc3d23e76be9118604dbeadc

gkotton/vmware-nsx,gkotton/vmware-nsx