Fix infloop in bidi.c

Fix infloop in bidi.c

* src/bidi.c (bidi_set_paragraph_end): Reset the isolate_level to
zero.  Whenever stack_idx is reset to zero, the isolate_level must
also be reset, since there cannot be any isolate status outside of
embeddings.  Failure to reset isolate_level will cause us infloop
when we see a PDI.  Reported by Matt Beshara <m@mfa.pw>.