I was inspired by a series of patches merged by Alan Modra in the other
projects, so I did the same in GDB with a bit of Coccinelle and grep.
This patch removes the unnecessary NULL checks before calls to xfree.
They are unnecessary because xfree already does a NULL check. Since
free is supposed to handle NULL values correctly, the NULL check in
xfree itself is also questionable, but I've left it there for now.