[cff] Adjust pre-allocation

This better matches actual usage, given that ops are one or two
bytes, and vector also allocates 50% extra.

src/hb-subset-cff-common.hh

@@ -565,7 +565,7 @@ struct subr_subsetter_t
       ENV env (str, acc, fd);
       cs_interpreter_t<ENV, OPSET, subr_subset_param_t> interp (env);
 
-      parsed_charstrings[i].alloc (str.length);
+      parsed_charstrings[i].alloc (str.length / 2);
       subr_subset_param_t  param (&parsed_charstrings[i],
 				  &parsed_global_subrs,
 				  &parsed_local_subrs[fd],