composite: validate warn_level using new WARN_LEVEL_MASK mask,

silently resetting to `WARN_DEFAULT` if not valid (ticket #359,
  props Andre Maute)

backend/composite.c

@@ -946,7 +946,7 @@ INTERNAL int zint_composite(struct zint_symbol *symbol, unsigned char source[],
 
     /* Hack to initialize `warn_number` to warning (if any) returned by `zint_gs1_verify()` */
     warn_number = symbol->warn_level >> 8;
-    symbol->warn_level &= 0xFF;
+    symbol->warn_level &= WARN_LEVEL_MASK;
     assert(warn_number < ZINT_ERROR);
 
     /* Perform sanity checks on input options first */