pack.c: remove wasted allocation in BER integer packing

The 'w' format (BER compressed integer) was allocating an empty
string with rb_str_new(0, 0) then immediately overwriting it with
the correctly-sized allocation. Remove the wasted first allocation.

~50% improvement on BER pack benchmarks.

Author: khasinski

pack.c

@@ -736,7 +736,7 @@ pack_pack(rb_execution_context_t *ec, VALUE ary, VALUE fmt, VALUE buffer)
 
           case 'w':		/* BER compressed integer  */
             while (len-- > 0) {
-                VALUE buf = rb_str_new(0, 0);
+                VALUE buf;
                 size_t numbytes;
                 int sign;
                 char *cp;