Fix filter valid/invalid test for multifield custom type

Check for "or" or "||" (also ignore whitespaces)
and check both arguments of this operator to be
fieldname (character check), then check whole filter is valid
for dfilter compiler.

Change-Id: I412b5dc1fca16fcd2b640aa74af81167300fd11e
Reviewed-on: https://code.wireshark.org/review/5848
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Tested-by: Michal Labedzki <michal.labedzki@tieto.com>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>