tools: remove unnecessary null pointer checks before frees

tools: remove unnecessary null pointer checks before frees

Patch generated by the following semantic patch
(http://coccinelle.lip6.fr/):

@@
expression *P;
@@

- if(P) free(P);
+ free(P);

...and then by filtering through the following command:

filterdiff -p1 -x 'stubdom/*' -x 'tools/firmware/*' -x 'tools/qemu-*' -x 'tools/blktap*'

Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>