fix segv in regcomp.c:S_join_exact()

fix segv in regcomp.c:S_join_exact()

This function joins multiple EXACT* nodes into a single node.
At the end, under DEBUGGING, it marks the optimised-out nodes as being
type OPTIMIZED. However, some of the 'nodes' aren't actually nodes;
they're random bits of string at the tail of those nodes. So you
can't peek that the 'node's OP field to decide what type it was.

Instead, just unconditionally overwrite all the slots with fake
OPTIMIZED nodes.