QemuOpts: Fix qemu_opts_foreach() dangling location regression

qemu_opts_foreach() pushes and pops a Location with automatic storage
duration.  Except it fails to pop when @func() returns non-zero.
cur_loc then points to unused stack space, and will most likely get
clobbered in short order.

Clobbered cur_loc can make loc_pop() and error_print_loc() crash or
report bogus locations.

Affects several qemu command line options as well as qemu-img,
qemu-io, qemu-nbd -object, and blkdebug's configuration file.

Broken in commit a4c7367, v2.4.0.

Reproducer:
    $ qemu-system-x86_64 -nodefaults -display none -object secret,id=foo,foo=bar

main() reports "Property '.foo' not found" like this:

    if (qemu_opts_foreach(qemu_find_opts("object"),
                          user_creatable_add_opts_foreach,
                          object_create_delayed, &err)) {
        error_report_err(err);
        exit(1);
    }

cur_loc then points to where qemu_opts_foreach()'s Location used to
be, i.e. unused stack space.  With optimization, this Location doesn't
get clobbered for me, and also happens to be the correct location.
Without optimization, it does get clobbered in a way that makes
error_report_err() report no location.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1461767349-15329-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

util/qemu-option.c

@@ -1108,19 +1108,19 @@ int qemu_opts_foreach(QemuOptsList *list, qemu_opts_loopfunc func,
 {
     Location loc;
     QemuOpts *opts;
-    int rc;
+    int rc = 0;
 
     loc_push_none(&loc);
     QTAILQ_FOREACH(opts, &list->head, next) {
         loc_restore(&opts->loc);
         rc = func(opaque, opts, errp);
         if (rc) {
-            return rc;
+            break;
         }
         assert(!errp || !*errp);
     }
     loc_pop(&loc);
-    return 0;
+    return rc;
 }
 
 static size_t count_opts_list(QemuOptsList *list)