fix segv in regcomp.c:S_join_exact()

fix segv in regcomp.c:S_join_exact()

[ cherry-picked from bb789b09de07edfb74477eb1603949c96d60927d
to stop clang's address-sanitizer from complaining. See [perl #115994] ]

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.