qcow2: fix null pointer dereference

It seems this assert() was somehow misplaced.

block/qcow2-refcount.c:2193:42: warning: Array access (from variable 'on_disk_reftable') results in a null pointer dereference
        on_disk_reftable[refblock_index] = refblock_offset;
        ~~~~~~~~~~~~~~~~                 ^

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>