labelsfilter: Improve sanitization
labelsfilter: Improve sanitization

OSS-Fuzz reports that the labels sanitization for the --labels option
doesn't properly check the input from the user. Add some extra basic
checks for naive bad inputs.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium