Fix fatal warn when compiling under GCC 5.2.0

GCC 5.2.0 generates the following [fatal] warning:
dialog_util.c:270:23: error:
zero-length gnu_printf format string [-Werror=format-zero-length]

sprintf(dargv[n++], "");

Fix malloc argument while here, removing sprintf.

Reported by: Ruslan Bukin <ruslan.bukin at cl cam ac uk>