tracker-needle: Fix crash in result store
tracker-needle: Fix crash in result store

n_children() would crash if there weren't any items
in the categories array. Fixes GB#661446, reported by
Damien Gombault <desintegr@gmail.com>

hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker