devtools: add script to fix unnecessary null checks

devtools: add script to fix unnecessary null checks

This script is based on the idea of the nullfree script
in the Linux kernel. It finds cases where a check for null
pointer is done, but is unnecessary because the function
already handles NULL pointer.

Basic example:
       if (x->buf)
           rte_free(x->buf);
can be reduced to:
       rte_free(x->buf);

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>