High: fix infloop when no schema validates on update_validation

Test case:
$ ./autogen.sh; ./configure; make # if not already
$ sed -i.orig 's|"promote"|"infloop"|' \

pengine/test10/use-after-free-merge.xml

$ PCMK_schema_directory=$(pwd)/xml \

tools/crm_simulate -x pengine/test10/use-after-free-merge.xml -S

BEFORE:
Entity: line 54: element rsc_colocation: Relax-NG validity error :
Invalid attribute then-action for element rsc_order
Entity: line 49: element rsc_location: Relax-NG validity error : Element
constraints has extra content: rsc_location
[...infloop...]

AFTER:
Entity: line 54: element rsc_colocation: Relax-NG validity error :
Invalid attribute then-action for element rsc_order
Entity: line 49: element rsc_location: Relax-NG validity error : Element
constraints has extra content: rsc_location
[several times]
<error message>