skel_instance_of: fix bad check for regexp match
skel_instance_of: fix bad check for regexp match

The return value of regexp_match was interpreted incorrectly, making it
seem that the skel_instance_of test should fail if we ever tried to match
an empty string.

bkearney/augeas,camptocamp/augeas,dafugg/augeas,jjlin/augeas,lutter/augeas,kunkku/augeas,manandbytes/augeas,ptoscano/augeas,dafugg/augeas,bkearney/augeas,pevalme/augeas,kumy/augeas,ptoscano/augeas,ptoscano/augeas,lutter/augeas,jjlin/augeas,lutter/augeas,mlichvar/augeas,domcleal/augeas,mlichvar/augeas,mlichvar/augeas,domcleal/augeas,hercules-team/augeas-do-not-use,hercules-team/augeas-do-not-use,hercules-team/augeas-do-not-use,mchf/augeas,kumy/augeas,bkearney/augeas,pevalme/augeas,raphink/augeas,jasperla/augeas,hercules-team/augeas,GeoffWilliams/augeas,jasperla/augeas,bkearney/augeas,dafugg/augeas,MikaelSmith/augeas,camptocamp/augeas,jtopjian/augeas,camptocamp/augeas,camptocamp/augeas,kunkku/augeas,domcleal/augeas,kumy/augeas,raphink/augeas,hercules-team/augeas,pevalme/augeas,jjlin/augeas,kunkku/augeas,GeoffWilliams/augeas,mchf/augeas,MikaelSmith/augeas,manandbytes/augeas,jtopjian/augeas,kumy/augeas