Protect the bidi iterator against zero bidi properties.

src/bidi.c (bidi_get_type): If bidi_type_table reports zero as the
 character bidirectional type, use STRONG_L instead.  Fixes crashes
 in a buffer produced by `describe-categories'.

src/ChangeLog

@@ -1,5 +1,9 @@
 2011-08-18  Eli Zaretskii  <eliz@gnu.org>
 
+	* bidi.c (bidi_get_type): If bidi_type_table reports zero as the
+	character bidirectional type, use STRONG_L instead.  Fixes crashes
+	in a buffer produced by `describe-categories'.
+
 	* dispextern.h (struct bidi_it): Move disp_pos and disp_prop_p
 	members before the level stack, so they would be saved and
 	restored when copying iterator state.  Fixes incorrect reordering

src/bidi.c

@@ -108,6 +108,8 @@ bidi_get_type (int ch, bidi_dir_t override)
     abort ();
 
   default_type = (bidi_type_t) XINT (CHAR_TABLE_REF (bidi_type_table, ch));
+  if (default_type == 0)
+    default_type = STRONG_L;
 
   if (override == NEUTRAL_DIR)
     return default_type;