[ot-font] Work around broken cmap subtable format 4 length

Roboto was hitting this.  FreeType also has pretty much the
same code for this, in ttcmap.c:tt_cmap4_validate():

    /* in certain fonts, the `length' field is invalid and goes */
    /* out of bound.  We try to correct this here...            */
    if ( table + length > valid->limit )
    {
      if ( valid->level >= FT_VALIDATE_TIGHT )
        FT_INVALID_TOO_SHORT;

      length = (FT_UInt)( valid->limit - table );
    }

src/hb-ot-cmap-table.hh

@@ -131,11 +131,25 @@ struct CmapSubtableFormat4
     return true;
   }
 
-  inline bool sanitize (hb_sanitize_context_t *c) {
+  inline bool sanitize (hb_sanitize_context_t *c)
+  {
     TRACE_SANITIZE (this);
-    return TRACE_RETURN (c->check_struct (this) &&
-			 c->check_range (this, length) &&
-			 16 + 4 * (unsigned int) segCountX2 < length);
+    if (unlikely (!c->check_struct (this)))
+      return TRACE_RETURN (false);
+
+    if (unlikely (!c->check_range (this, length)))
+    {
+      /* Some broken fonts have too long of a "length" value.
+       * If that is the case, just change the value to truncate
+       * the subtable at the end of the blob. */
+      uint16_t new_length = (uint16_t) MIN ((uintptr_t) 65535,
+					    (uintptr_t) (c->end -
+							 (char *) this));
+      if (!c->try_set (&length, new_length))
+	return TRACE_RETURN (false);
+    }
+
+    return TRACE_RETURN (16 + 4 * (unsigned int) segCountX2 <= length);
   }
 
   protected: