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>.

src/bidi.c

@@ -1126,6 +1126,7 @@ bidi_set_paragraph_end (struct bidi_it *bidi_it)
   bidi_it->invalid_levels = 0;
   bidi_it->invalid_isolates = 0;
   bidi_it->stack_idx = 0;
+  bidi_it->isolate_level = 0;
   bidi_it->resolved_level = bidi_it->level_stack[0].level;
 }