qdev: Fix memory leak in function set_pci_devfn

Valgrind reported this memory leak which occured very often.

Test scenario:

qemu-system-i386 (no arguments), only BIOS started, terminate with
monitor command (quit).

v2:
Use error_free instead of g_free (hint from Andreas Färber, thanks).
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Acked-by: NAndreas Färber <afaerber@suse.de>

hw/qdev-properties.c

@@ -834,6 +834,7 @@ static void set_pci_devfn(Object *obj, Visitor *v, void *opaque,
 
     visit_type_str(v, &str, name, &local_err);
     if (local_err) {
+        error_free(local_err);
         return set_int32(obj, v, opaque, name, errp);
     }