Remove useless NULL check for qemu_strdup return value

Found with this Coccinelle semantic patch:
@@
expression E;
identifier ptr;
identifier fn ~= "qemu_strn?dup";
@@

-ptr = fn(E);
-if (ptr == NULL) { ... }
-
+ptr = fn(E);

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
