qemu-img: initialize MapEntry object

Commit 16b0d555 introduced an issue where we are not initializing
has_filename for the 'next' MapEntry object, which leads to interesting
errors in both Valgrind and Clang -fsanitize=undefined.

Zero the stack object at allocation AND make sure the utility to
populate the fields properly marks has_filename as false if applicable.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
