In the linked reproducer, m->fields_size == 0, and we calculate ri == -1, which
of course doesn't end well. Skip the whole calculation if m->fields_size == 0,
and also check that we don't go negative even if it is non-zero.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19446 and #15583.
