Update pack.c

Co-authored-by: Nobuyoshi Nakada <[email protected]>

Author: tenderlove

pack.c

@@ -680,9 +680,9 @@ pack_pack(rb_execution_context_t *ec, VALUE ary, VALUE fmt, VALUE buffer)
                 numbytes = rb_absint_numwords(from, 7, NULL);
                 if (numbytes == 0)
                     numbytes = 1;
-                buf = rb_str_new(NULL, numbytes);
-
-                sign = rb_integer_pack(from, RSTRING_PTR(buf), RSTRING_LEN(buf), 1, 1, INTEGER_PACK_LITTLE_ENDIAN | INTEGER_PACK_2COMP);
+                rb_str_modify_expand(res, numbytes);
+                cp = RSTRING_END(res);
+                sign = rb_integer_pack(from, cp, numbytes, 1, 1, INTEGER_PACK_LITTLE_ENDIAN | INTEGER_PACK_2COMP);
 
                 /* Check if we need an extra byte for sign extension */
                 last_byte = (unsigned char)RSTRING_PTR(buf)[numbytes - 1];